If you see some lesser known skater in practice and their landing difficult triples , and their skating skills are up there with the skaters you expected to be on top. That's going to affect your marks in the competition. But if you go in there not knowing who said skater is, and not seeing that the skater can hold their own with the top skater, than your marks might be lower for said skater.
Practices allows the judges to literally see skaters side by side and compare them. And they get to see all the skaters not just the well known ones.
Practices gives the lesser known skaters an opportunity to impress the judges. Also maybe the skater who wasn't skating well earlier in the season, but is skating well know can also impress the judges too.
In contrast if the judges see that the well known skater is skating crappily in practice and also skates crappily in competition, they may be less willing to give the famous skater the benefit of the doubt.
Practices IMO actually level the playing field just a little bit.