Why do you use a smash shot in badminton?

you push the person into the back of their court, and once they are there, you drop it right in front of the net and they cant get to it

well actually, this isn't the case with professionals. They are quick through the court. They are always at their base position after a shot. The main thing about a drop shot is that it creates confusion. Your drop shot should never LOOK different from your clear. It keeps your opponent guessing.