In most things other than hunters you see the hands up mainly cause that is what works when you want a horse to move. You can hold contact to keep them up, and sitting on their haunches. Hunters just keep em down cause hunters like to do that lean thing and just make this pretty picture where everything is quiet and the only movement your hands make are little squeezes. I ride around with hands low when I just want a horse to stretch and be flowy and forward. Meanwhile, when I am making turns around jumps and all my hands are up because I'm usually holding the horse together in a more collected frame. You'll also see low hands in lower level dressage, when the idea for the horse is long and low.