In an ideal world, a team only needs its backup offensive tackle in a select few situations each game. He will enter the game on short yardage or goal line plays, and be a part of the field goal and extra point protection units. Other than that, he will sit on the bench and provide depth behind the starters that will never be needed.
Of course, the NFL is not an ideal world and the backup offensive tackle position has become immensely important. Just ask the New England Patriots, who saw their projected starters miss considerable time during the 2020 season: while right tackle Marcus Cannon opted out, left tackle Isaiah Wynn missed the final six games of the year after suffering a knee injury in Week 11.