Another reason they are good for assistance is that after a good, heavy squat session you may have stability issues that keep you from getting much from more squats. Leg presses are still a far more natural movement than leg extensions and other leg assistance options, and if you like doing burn out sets, like I do, you can take yourself right to the limit because of the inherent stability of the leg press. It doesn't get better than a few sets of 30-50 reps to finish off a leg/squat session.
Current PRs at 242, raw w/ wraps- 525, 355, 605, 1485
Roboro tui, perimo vester adversarius
(Build yourself, destroy your enemy)