What do they actually do?

 

You don't want to use longer stops, just limits your turning radius.

 

My wife's rover has the shorter bump stops and her 275 tires rub at full wheel crank, so in this instance one might want longer stops, we just don't crank it all the way.

 

They keep you from hitting the limit of your extended (or lowered) shocks. They could also be used to keep the tires from hitting the top of the wheel wells if that's happening.

 

I was thinking something similar, I have this issue where after I installed my 2 inch spacer lift and I hit a pretty descent bump in the road I have this very solid and loud banging noise. Later on I figured out it was my axle hitting the frame or something like that. I was wondering if longer bump stops would fix this.

 

Oops, I see know I was talking about the wrong stops. Sorry

 

No. That makes no sense. Your 2" spacers are pushing the axle 2" farther from the stock bumpstops. You extend your bumpstops so your tires(usually larger with a lift) don't rub the wheels arches or your shocks don't bottom out.
I'd check all the bolts on your suspension to find the banging sound.

 

before my lift I could hit any bump and be fine, after my lift I have to slow down for any bump now, me and my dad were both bouncing on the front of the car and we coould make it do the banging noise. While we were doing that my brother was below the car seeing what it was, he said it was the axle hitting the front bump stops or something like that.

 

are we confusing steering stops with bump stops

 

Do you have two inch spacers for a body lift or two inch spacers on your springs? If a body lift I'd say something isn't tight because that would not change your suspension. If a spring lift did you get longer shocks? I'm puzzled by this.