110 volt AC requires only a single pole breaker.

What are double pole 20a breakers used for?

All double-pole (regardless of amperage) are for 240/220V. There are two phases in each breaker panel, and they each carry 120V, and go every other pole. So when you have a double pole, you're tapping into both phases. 120V+120V=240V

I've seen double-pole breakers rated for 120 volts. SquareD's QO215b is rated for 120/240 and takes only one slot. I think, but am not sure, that the idea is to piggyback two circuits on one slot when you are running low on slots in your load center box.