If you mean the one on one with the CB, he's supposed to create separation at the LOS because it was 1 on 1. The back shoulder throw is done because the WR DOESN'T have separation, that's not the original plan the original plan is the WR beats Isaac Yiadom coming off the LOS and the QB gets him the ball.
Brady identified that exact matchup before the ball was snapped, but Yiadom had good coverage. It wasn't a great backshoulder throw for sure, but the idea is the Wr is supposed to win that matchup