why would you ever sunder anything?
Because you need the prereq for Combat Brute and figure "what the heck?"
Because you want to ruin a spellcaster's day without killing him.
Because you're the DM and you accidentally let your characters have too much loot.
Because it's low level and somebody's using a freaking tower shield to be unhittable.
Because you're Sauron and Isildur cut the Ring off your finger.
Because you're using an alternate rules system like Armor as DR, where destroying the Armor gets rid of the DR.
Because your opponent has a double-bladed lightsaber.
Because your opponent's source of power is his staff.
Because your opponent just used his holy symbol to turn your undead minions and the DM rules that breaking the symbol ends the turning effect.
Because you're fighting a hydra and chopping off its heads uses the sunder rules.