Not a silly question: I used to wonder exactly the same thing....

They are used when rotating the head. In the "-" position they provide click-stops (rather than an actual lock) to centre the head vertically. In the "O" position, the click-stops are disengaged to allow easy rotating of the head.

At least, that's in my enlarger - it is possible that they are supposed to lock and that mine are damaged....

Cheers, Bob.