well, actually they probably do a bit more than that.
They have to take the order, package up the order instead of just running the plate out, make sure it's bagged properly, add condiments, which probably have to put in little tiny containers & make sure the timing is right. They are probably doing the whole order themselves. At a fast food place it's done by everyone.
I used to hate doing take out orders when I worked in a restaurant. It's a PITA. I would at least tip 15%, usually there are no drinks, so the tab wouldn't be as much as it would if you dined in.