It's a combination of
- personal growth
- meeting people, esp other speakers (== ROI infinity)
- scoring leads
- building credibility / personal brand
Want to get more public speaking gigs? Instead of waiting harder, do this:
Have a high quality recording of a talk of yours. Don't have one? Do a talk to your friends and record it (needs to be high quality)
Take note which events open a public call for speakers. A lot of them do. Apply. You can set Mention alerts for each time a conference mentions "call for speakers".
Identify the organizers. Start engaging with them on social (LinkedIn, Twitter), so you get yourself on their radar.