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...if the dropouts are really long, I use the screws to either set the wheelbase longer or shorter, or sometimes to allow for a fatter tyre if moving the wheel way back will help with fender and stay clearances.

Otherwise, I just mostly use them for fine tuning a centered position for the wheel alignment. And I'm not sure I ever change it much once I have a particular bike set up. I'm working on one of those Gios Compact frames now, with adjustable dropouts. The rationale for those was you could adjust the wheelbase and thus your handling for a particular style of racing. (you probably already know this)