Reason #1: Will-O-Wisp. Don't wanna be completely shut down by it would you?
Reason #2: For those physical walls that think they can wall Tyranitar and his monstrous base 134 atk, fear not because Ice Beam is here!
Reason #3:To surprise the opponent. Nobody would expect either but most people would expect Ice Punch with base 134 and send in a physical wall.
Reason #4: As you said, 95 base sp.atk isn't too shabby so it is usable and more powerful.
I'll edit in more IF I find any.
Hope this helps!
-It's a secret!