The little hook on the inside of a car door is usually called a coat hook or grab handle hook (depending on the design).
It’s mainly there for simple practical reasons:
- Hanging clothes like a jacket or blazer so they don’t wrinkle while you’re traveling
- Holding light items like a small bag or hat
- Sometimes it’s part of the assist handle area, so it’s built into the same structure people use to steady themselves when getting in or out of the car
It’s not meant for anything heavy—just light, temporary hanging.
If you meant something more specific (like a certain “hook” feature in a particular car model), tell me which car and I’ll break it down more precisely.