Having the last action is a powerful advantage. The main advantage is information, every action that anyone takes defines their range a little more. When villain is OOP your range is wider than if you'd already acted and that's tough for him to play against, he's also got to be aware that any action he takes will give you a better idea of his range so he has to be concerned about being bluffed off of his hand or value towning himself. I use the example of an opponent being OOP, because you don't want to be in that situation any more than you have to be.
Consider also that the pot is biggest on the river, and because of that the bets will be biggest then, at that point there is a very big advantage in getting to choose to value bet or check behind. That's huge, don't underestimate it.
My general rules of thumb:
When OOP, I'm never playing marginal situations without a solid read.
OOP I just call much less than 1% of the time, fold or raise. There are a ton of reasons behind this, mostly just that you should favour fold
equity and raising gives a playable SPR compared to calling which highly favours the guy in position... suffice to say, calling is suicide and raising isn't terrible but neither is good because of position.
I'd rather open raise 72o from the button than raise ATs UTG.
