Upon pondering the matter, it occurs to me that I’ve never seen a short stocky guy do this, it seems to be more a habit of tall lanky dudes. It certainly does change the angle a bit, which would be more important for a tall guy, but I think it’s mosty for the 3-point stability. A short guy can plant himself in a wide-legged stance and get sufficient 2-point stability, but if a tall long-legged fellow does this he’s edging over into the territory of the guy at the next urinal.
When this subject came up, I immediately thought of my late father-in-law who was a chronic hand-on-the-wall type (not that I was particularly into watching him pee, you understand, but I travelled with him a lot and stood beside him at countless rest-stop urinals). He was one of these laid-back types that could never walk a straight line or stand at attention. He ambled when he walked, rambled when he talked and usually found something to lean against when he stood. If he had to free-stand, he shuffled and swayed…nothing wrong with him, just his nature. But when answering a call of nature, he definitely needed that third stability point. If he hadn’t had it, it would not have been safe to stand next to him…the stream would have gone everywhere!