I’ve had quite a few people ask me to explain Canto Bight because its importance in the story wasn’t immediately apparent to them, so I hope you’ll strap in for this.
One of the most important themes of The Last Jedi is that of failure. We don’t learn from it enough and we don’t expect it when maybe we should.
Poe is the one this naturally centers on the most as it’s the major plot of his arc. At the beginning of the film, he’s standing alone against the First Order and ignores Leia’s orders to take down a Dreadnaught.
We don’t question him, even though he turns his back on Leia. Because he’s a character we trust and in Star Wars, we’re used to seeing the characters with the most ridiculous plans win. Every time.
This is something that happens throughout all the films. From Qui-Gon betting on the podraces to Han slicing his way into the Endor bunker: bad plans with big risks always seems to work.
With Poe and the Dreadnaught, this plan works for us as an audience. We’re slapped in the face by Leia as much as Poe is when she declares that mission, despite its success on paper, as a failure.
“There were a lot of heroes on that mission.”
“Dead heroes. No leaders.”
“Dead heroes. No leaders.”
Holdo then takes command after the loss of Resistance High Command and gives us a speech that explains what we’re looking after: the MOST IMPORTANT MISSION OF THE RESISTANCE is to spread the spark of hope.
That’s it. That’s the mission. They need to live to spread that hope to others because the few hundred of them left can’t possibly fight the First Order alone.
That spark of hope brings us to Finn and Rose and how they meet. Finn hasn’t grown as a character. He hasn’t really picked a side, he just wants to save Rey. He didn’t go to Starkiller to become a hero, he “just wanted to save Rey.”
That he got draped in the trappings of a hero is disconcerting to him. At the point where the First Order can track them in hyperspace and it’s a waiting game until they’re blown out of the sky, he doesn’t want Rey to come back to this.
Which is why when he’s able to get his hands on Leia’s cloaked binary beacon that will lead Rey to them, he doesn’t hesitate to come up with a plan to jump ship. Because this will keep Rey safe.
Instead of getting away, he gets tazed by Rose Tico. A mechanic who just lost her sister in Poe’s bombing run on the Dreadnaught. She’s going to toss him in the brig until they realize they cracked the code on how to disrupt the active hyperspace tracking.
Naturally, they take this information to Poe whose instinct is to want to blow up the lead ship. When they explain why that won’t work, they come up with the idea of sneaking aboard Snoke’s ship.
Again, Poe still has not learned his lesson of leadership. When told that Holdo would never approve such a risky plan, he agrees and his solution is, “Fine, we just won’t tell her.”
Maz Kanata is the only person they know to ask for who they can trust to get them the codes to get aboard the destroyer, but she’s occupied. Plenty of people are qualified to do this job, but there is only one SHE TRUSTS.
The Master Codebreaker, played by Justin Theroux, will be in the casino at Canto Bight wearing a red flower on his lapel. (which looks very much like Indy’s tux in Temple of Doom). He is not the only person capable of the job, but, like I said, the only one Maz trusts for them. 
So they set out to do it. Poe uses subterfuge and his contacts on the bridge to conceal their departure from Holdo and the Admiralty and they’re on their way.
Canto Bight itself is but two sequences offering us a different view of the galaxy. It expands the moral complexity of universe and gives us a window to better understand Finn and Rose.
The film takes cues from Hitchcock films like To Catch a Thief and Bond Films set in Monte Carlo. The fact that the mechanic and ex-stormtrooper are there creates a disconnect.
It's another interesting parallel, too, that Attack of the Clones adopted much the same structure as a Bond film, casting Obi-Wan in that role, further reinforcing the rhyming structure of the middle chapters of the trilogies.
They should have sent a spy, someone who could blend in, rather than the pair of kids who were going to park their shuttle on the public beach and wander directly to the casino. This further illustrates Poe’s bad judgment.
But this also serves to show us the differences between Finn and Rose. Finn hasn’t seen anything in the galaxy and is completely taken by this place. But Rose comes from an almost harder background and see the hurt it provides.
She shows him those suffering beneath the surface. The stable hands and the animals. And gives us one of the best, pulpy lines in the film. “I wish I could put my fist through this whole lousy, beautiful town”
This is a side of the galaxy we’ve never seen. The oligarchs who made their money with the military industrial complex. With the galaxy at war and the Republic destroyed, none of these people seem hurt by this. They’ll probably all MAKE money on the political status quo.
And since Finn and Rose are bad at this sort of mission, it’s up to BB-8 to find the Master Codebreaker. Once he alerts Finn and Rose of his presence? They blunder into the casino, completely out of place, and get arrested.
They get arrested for a 27-b-stroke-6, which is a wonderful reference to Terry Gilliam’s Brazil. But they’re arrested for parking their shuttle on the beach. They’re tazed and sent straight to the drunk tank. 
Now, the drunk tank is where we meet DJ. He’s A codebreaker. And not one we trust. He’s Finn in 10 years if he develops a skill and insists on not joining anyone.
Naturally, they don’t trust him and would rather find their own way out of prison and they find their way to the fathier stables.
The fathiers are a pretty 1 to 1 analogue for horse races and these fathiers are as abused as the stable kids who take care of them, but this moment where Finn and Rose come in contact with the stable kids might be one of the most important in the film.
They reveal their symbol (Which Holdo explained the oppressed across the galaxy recognize) and are able to get the kids to help them escape.
We get a wonderful sequence that evokes the best of Spielberg from the 80s, that childlike magic and wonder of hope, set to John Williams music to match, that shows us how satisfying it is to watch those with too much have their toys broken.
When hope looks lost for them, who do they have left to help them? The nihilist and BB-8 who arrive in a stolen ship. Then we’re off of Canto Bight and on our way to the next phase.
The film sets up an angel on Finn’s shoulder and a devil. The angel is obviously Rose, who shows him how important empathy is and why doing the right thing regardless of reward is correct. The devil is DJ, who literally only works for self-motiviation.
He explains that he’s not bad or good, just in it for himself, which is the entire premise of Canto Bight. These people have fooled themselves into thinking they’re above the fray. It’s doubly illustrated by the arms dealer whose ship they stole, selling weapons to both sides.
Finn isn’t a Jedi and doesn’t get a test in a mirror cave, but Canto Bight comes as close as he can get to that. He can become either someone like Rose or someone like DJ.
At this point, the situation for the Resistance is getting more helpless and Poe decides to cause a scene and find out what’s going on.
After yelling at his superior officer, he stumbles onto a piece of the plan. “We’re abandoning ship?” He then accuses Holdo of being a coward and a traitor and immediately runs to tell Finn how dire the situation is.
That he does this is EXACTLY the reason he should not have been privy to any details of the plan. He tells Finn, while DJ is in earshot, that they’re loading everyone up into transports to get away from the FO. We get a close up of DJ listening to this.
They arrive on the Snoke’s ship and everything goes to hell. Poe, wishing to buy them more time for this bad plan, declares a mutiny against Holdo. Finn, Rose, and DJ, are caught by BB-9E and, over the comm, Poe realizes his plan was a failure right before getting tazed by Leia.
They toss Poe on a transport that should have left ages ago and he has to watch their escape in slow motion. With no hyperdrives and no shields.
And we learn, because of Poe’s lax operational security, DJ is able to cut a deal with the First Order and he tells them of the transports, that they begin picking out of the sky, one by one.
This is the price of failure. This is the price of Poe’s hubris. This is the price of Finn’s inexperience and selfishness. We didn’t expect it could happen. Neither did Poe. But it did.
Poe finally understands how badly he screwed up as he watches Holdo sacrifice her life to pay for his mistakes. And none of that would have had any meaning without Canto Bight.
This failure plays a further role as well. Both Finn and Poe learn their lesson here, but that lesson is at odds. Finn learns that there are things worth sacrificing his life for, but Poe learns that it should only be done when there’s meaning behind it.
Rose drives the point home with her second beautiful, breathtaking line, “I saved you, dummy. That’s how we’re going to win. By saving the things we love, not destroying the things we hate.”
But it pays off even further and comes full circle in two ways. First, when Luke comes to make his standoff, it is Poe who recognizes it for what it is. Because he began the movie standing alone against the First Order, he knows it’s a diversion.
The second way it pays off goes back to Holdo’s speech. Think about her words as we go back to Canto Bight and see how this legend inspired these stable kids. They know the symbol of the Rebels. They know they aren’t alone.
The spark lived in the face of failure.
Hope survived and spread because of one connection made in the stables of a horrible city like Canto Bight.
So, someone can tell you they didn't like Canto Bight. That's valid. What they can't tell you is that it was pointless. Because it wasn't.
It added to the theme, the character development, complicated the plot for the climax getting the Resistance to Crait, aided the characters in learning their lessons for the finale AND served as the site for the most important epiloge.
