I think it just feels good, honestly. I've definitely done it. Perhaps some try to use it as a way to convince themselves someone's riding them or maybe they're being fantasy involves being their asses or back being rubbed or brushed against something.
Any other movements? Uh, well... I sometimes work myself into other positions, whether it's legs up against the wall or humping a pillow/the bed, sometimes it's to feel more of a breeze in/on/around some areas. Anything to make it feel more than just incredible. I'm not one of those 2-minute types that rush through it, I like to play for a bit.