Not a stupid question at all, especially if you're recreational or new, or doing things for sports like football. Well, it depends in part on which coach you ask, but in general:
Power variations are focused more on explosive power generation. You can't pull as heavy as a competition style clean or snatch, but the power output is higher and some say it trains better extension if that is a problem for a lifter.
Hang variations can work isometric strength at the angles you hang at. This is assuming you are not doing a quick dip down and rebound into the lift like most football programs (as an example you can youtube videos of Klokov doing pause snatches or pause cleans from the hang). They may also be used to teach lifters how to feel their balance and body--in other words to train them to feel the right bar path at particular points. Kind of like checkpoints through the lift. They can also be used to help the lifter get under the bar faster when using a full lift from the hang instead of a power lift from the hang (full snatch from hang above the knee vs. power snatch at hang above the knee)
Blocks are typically used for starting strength or acceleration from the point in the lift that the bar starts at.
In general lifts that are shorter than normal--either because they are starting from the hang or blocks, or they are the power version of a lift, or both--are typically much easier technically speaking than the full lifts from the floor, so they can be used as teaching tools. They also tend to induce less fatigue so can be used if a lifter needs a break from the physical strain of a particular training block while also not skipping training the movement itself--the movements need to be practiced frequently for competitors to retain the proper coordination and bar path, even if it's lighter or not the full competition lift.
Those would be reasons off the top of my head.