To me, it looks like mostly flavor. Physical connection between the attacker and target so that the disruptive energy can "resonate".
It could also be a balance thing, in some respects. Losing 1d6 of your own hit points is a little more significant when you're in hand-to-hand combat than it is if you're 200 feet away, tucked in a tree, with a few healing potions at hand you can drink at any point without worrying about attacks of opportunity.