After a bunch of googling around, I haven’t found an authoritative answer to my question: how are you supposed to set up and use your refrigerator’s food crisper—fruits and vegatables need different levels of humidity (which ones need what I wasn’t sure), and the little baffles are supposed to change that (which setting does what I wasn’t sure).
So it seems that they are more about humidity than they are about keeping your foods fresh.
That being said, this Cooks Illustrated PDF seemed to give a half-way decent explanation of the proper settings and use:
Opening the baffles for air to pass through lowers the humidity, which is fine for basically anything that needs to be kept cold (like some fruit: apples and grapes)
Closing the baffles to stop air from circulating allows the humidity to rise for leafy vegetables.
But according to Cooks Illustrated, it’s all much more complicated than that—some produce like it warmer than the crisper may provide, like green beans, subtropical fruits, melons and herbs. So good luck if you have roommates.