The user has a symbiotic relationship with whatever life forms (insects, worms, snakes, etc.) they are sharing a body with. The insects get food from either the user or from whatever the person wants them to devour, and they use them for their needs.
In several cases, the user is actually a hive-minded swarm of the creatures.
Using insects they can lift things. Several thousand could lift and subdue targets; millions could lift a vehicle.
With the poisons most insects produce as a means of defense, they can easily poison and kill people with millions of bites or stings. By directing insects down the throat, they could easily suffocate a victim. Even a few ants down the throat could cause suffocation due to the small number of stings closing the windpipe. By swarming thousands of bees, hornets or wasps the target can be stung to death.
With the use of thousands of locusts or caterpillars, they can devour surrounding plant life.
In the event of being composed of insects, the user is effectively intangible, allowing attacks to simply pass through them.
With any insect of small size (so that means most insects) they can scout areas. The insects will report what they find to the "hive".