Maybe we're bringing Mannion in expecting him to end up being QB3.
At surface value Mannion gives Sloter a veteran to compete against and prove he's what we think. If he has already "earned" the backup spot he'll continue to display it in his play. He really needs to keep showing stuff as it is anyway. We couldn't give him the #2 spot and afford any bad preseason games.
Mannion keeps the door open in a way where if it doesnt work out with Sloter we still have an experience player to also evaluate.