The Bite-Sized series of articles is meant to take the most powerful cards in Dominion and give a short explanation of what makes them powerful.
Witch is the first junker Dominion ever knew, coming from the base set, and to this day is one of the most powerful. The combination of giving out a Curse and being a halfway decent card to put into a deck is what makes Witch powerful.
Junking is a very strong effect in Dominion, strong enough that even if the Curses can be trashed, you still usually want to go for Witch along with the trashing just to slow your opponent down.