The effect that it has for me is that the tree seems to move all over the place haphazardly, which is confusing. Also, I like to focus on my next move, so after I buy an upgrade, I look to see which upgrade(s) I want next, and position the tree in such a way that I can reach as many upgrades as possible, without having to move the tree too often. And as the game can't reasonably anticipate my next move, it makes way more sense to let me move the tree myself.