In the NFL if you want to win you have to be able to plug at least some of the holes in your roster with key free agent signings. Many times those free agents are your own players. For whatever reason the team did not re-sign the player before their contract ended. Now they must decide if they truly want to let the player walk or bring them back. The problem is that sometimes when you wait up until the contract expires you send up overpaying.
The biggest example is recent history was former Panthers defensive end Charles ‘Big Money’ Johnson.