Changing tempo is a huge way to stress the defense. Once a defense gets set in your rhythm, they can adapt to it.
Many people think that tempo stresses a defense. They will blindly say that the faster we go on offense, the more stress it puts on a defense.
While there is a small grain of truth to this, and that a faster offense will make the defense go more plain and vanilla, I always revert back to a conversation I had with the best defensive coach I have ever met.
He told me that when an offense goes with high tempo it does force him to be more basic, but it also allows him to get in a rhythm and allows the players to get a feel for the game.