Hanging is only as important as you make it to be. You can gain just fine without incorporating hanging into your routine. It can also be a great addition to compliment other exercises, as well as a beneficial tool to break through a plateau.
Benefits of hanging are the same as stretching. It's all about elongation, and time under tension. Hanging helps to stretch the ligaments, and has also been known to bulk up base girth.
I would advise starting with a very low weight if you aren't already doing some sort of intense stretching. Plenty start out with 1lb, and increase weight by the pound over time.
As far as how much can be gained through hanging... well, just like everything else in PE, it varies from person to person.