Various degrees of convolution:
-You want to empty the Gold pile for some reason
-They're playing some actions engine and you want to gum up their deck (probably counts as convoluted since they'd probably have to have gotten the Tunnel from Masquerade) They don't have to reveal
-You're about to possess them near the end of the game, and they probably have +cards in their hand.
-They're running Wishing Well and have a lot of discard, so you figure they'll probably be able to discard it on their turn anyway, and you want to deny a free +2 cards.
-You know they have Witch in hand and those are the last two cards in the draw, or some other cycling situation.
-You know they have Crossroads (and some other draw or more Crossroads) in hand
-You know they have Scout and discard in hand.
-It's a 3 player game and you want to help them since you're ahead and they're doing the worst, to solidify your lead in Provinces/Colonies
-You're pretty much screwing them with pirate ship and trashing most of their golds. Similarly, with Thief/Noble Brigand