It also makes the opposing team spend time preparing for that specific package throughout the week, when they could be working on something else during that time. Say they spend 8-10 minutes a day on a 'Sully Package' during the week, you're talking almost an hour of prep time spent on that package, when Iowa might not even use it.
It gives the coaches and players more to worry about.
and you keep Sully around when he knows he's gonna be playing
and all it takes is an injury to QB1 (assuming it's not Sully) and Sully is thrown in there