Teams need a pecking order when they open camp, because they start out in 7 on 7s and 11 on 11s from day one. They use the off season, voluntary work outs, and the OTAs to learn the playbook, so that once camp opens up the players already have an understanding of what their playbooks entail, and an understanding of what the coaches expect of them. Teams need a set first, second, and third string so their drills and team sessions run fluidly.
But things can change.
The Chicago Bears were correct in naming Mike Glennon the starter, because he was signed to man that position, and he’s been with the team longer than the kid they drafted.