it changes the pedalling motion to something that's pretty much like you're stepping up on to the pedals like you're climbing a set of stairs. This use a slightly different set of muscles than seated pedalling which is good if you're getting tired.
it puts your whole body in a position where you can actually brace yourself against the bars to push even harder against the pedals. If it wasn't for the bars, the hardest you could ever push would be the same as your weight, but when you're simultaneously pulling the bike up while pushing down you can generate more force. A variation of this is when the rider tips the bike from side to side in synch with his pedalling.
