Drop out screw adjustment

[DaveM]

Older bikes with horizontal dropouts sometimes had more variation in how well the two dropouts were aligned. So if you just pull the wheel all the way back into the dropouts, it would end up at an angle in the frame. The screws let you set where the wheel sits when you pull it in so you don't have to tinker with alignment. As cyclerUK says, as far back as possible is a good general rule of thumb, though as long as the QR skewer is getting a good amount of dropout under it, it doesn't matter too much where it sits.