Table tennis is not just about attacking play. Backspin shots slow the game down and reset the speed that can build up in some topspin rallies. As backspin shots generally land short on the table, it can also prevent your opponent from hitting a fast shot which can allow you to recompose yourself.
If your player has a chop or chop serve unlocked, then there is even more of a reason to backspin as they can catch opponents out who might attack and then see the ball go into the net.