Tire width selection is a combination of personal preference as well as functionality.
The front tire typically benefits from a greater width as more surface area is in contact with the ground. This gives better grip during turns on loose ground. As well, a wider tire does absorb impact a bit more then a narrower one, as you mentioned.
The rear tire is not exposed to lateral forces as much as the front, so it doesn't benefit from a wider width as much (unless you climb a lot on loose terrain).
It's not an absolute science.
Oh, and your tires shouldn't be rubbing on anything - chain or frame.