Well no, you don't need torpedos. Especially on Battleships and half-decent cruisers like the Cleveland or the Aoba.
On the other hand, the -Kaze destroyers have guns which turn like mud and take forever to reload considering they're sub 155mm. They need their torps to hit.
Also, there's a certain amount of maniacal, evil genius pleasure in watching a while spread of torpedoes just obliterate someone