A: Extra training minutes from having 3 games in the off-season week, in which case you would field your trainees instead of the tank squad.
B: Not liking your chances of a sweep, in which case I still would have put a good squad out just in case the RNG does something silly and the opponent tries to sneak a playoff normal in a must win game.
Though in hindsight, I realize that the point of the tank squad was to guarantee no injuries.