These are unguided rockets used as terror weapons against civilian targets. They're not very accurate in the first place, so even if a defense system deflects a rocket slightly it can still land in a populated area. I suspect what they're actually trying to do is cause enough airframe damage to make the rocket break up in flight.
I assume that even if you don't burn through a descending missile, merely deforming the nose cone can still cause it to go off course.