It's a question of mathematics and physical sciences.
The main reason is to avoid the inertness of a mass over the horizontal straight line (between front and rear wheels axle) ... which is supposed to amplify the hopping movment.
Reason why there's a link bar between the crankcase and the caliper bracket !!!
But I'm 100% convinced that we're not able to feel the consequences on open roads when we ride.
Thus, I confess the most part of us who make modifications on bike is dictated by the design and the look, only !!!