Normally, then NFL Combine does not effect the top 10 or so players in the draft. Those players have done enough to prove they can play exceptionally well at the next level.
It's players like Dorin Dickerson of last year's draft that the combine is used for. Dickerson had a killer combine and went in the seventh round to the Texans.
Some players are right on the edge of being a first day choice. The difference could be huge in terms of contract money.
These are the players that can improve by how well they do at the NFL Combine.