I don't know why the Brummbär has it.
It helps with its pathing. It is quite expensive and only fires in one direction. Unlike tank destroyers, its targets, infantry, can be not as accessible to medium crush. Tanks have turrets so it matters little on how they approach their target in order to allow their gun to fire. In a lot of maps, the Brummbär would have trouble moving around in seeing how it has poor accelerations and rotation. Additionally the Brummbär is not the most accessible thing when comes to teching.