Venom/poisons are used to either help something eat, or keep something from being eaten.
During mating (2:10 in this video), the male stings the female.
This species doesn't use venom to hunt, so the exact reason behind this behavior is unknown.
For example, sometimes ants will deliver formic acid straight into bite wounds. Sometimes, they just spray it everywhere.
1/2) Detection/Identification-finding the prey
3.) Approach-getting close to the prey
4.) Overpowering-subduing the prey
5.) Consumption-actually eating the prey
Venom or poison can be used at any of those last three stages
Jellyfish can do something similar...releasing clouds of nematocysts into the water around them.
Ampulex compressa appears to put it's prey into a very deep sleep to let it's larvae feed.
In fact, in parasitic wasps, the venom is used to subdue the prey by disrupting the host's immune system. In some species, there's no outward indication of the sting.
They coat these webs in oxalic acid, which dissolves the exoskeleton just enough to cause their prey to die of dehydration.
Venom can be used to make an animal avoid something, and poison can be used to make it sick.
Most predators can learn to avoid 'bad' things.
First, some insects contain compounds which can irritate sensitive membranes. Paederus beetles, a tiny orange rove beetle, can destroy skin with it's chemical defenses.
This hornworm feeds on toxic plants, and stores the toxins it eats to defend itself.
It's also thought that venom is introduced through an injury (bite/sting) and poison is eaten or smeared.