Pre workout, I think it’s valid to do some smashing and mobs in order to take you from stiffness to the ability to at least get into position.  If you are tight and jump into a workout requiring you to get to a good position, you could hurt yourself.  So you can think of smashing and mobs as a warmup and also as an evaluation on how well can you get into the archetypal shapes. 

If you smash too much pre-workout, you could cause some trauma which could limit your workout.

Post workout, as some have said here, you are trying to work out the kinks and tight spots and induce a parasympathetic response by cooling yourself down and stopping muscles from being in a perpetually post workout tight state which can cause soreness, etc.