Usually playing Goons leaves you with a dilemma: should you weigh down your deck with Coppers to get extra VP chips? Watchtower solves this problem neatly by trashing the Coppers as they come into your deck, even while you still collect VP chips for them.
Watchtower is also useful when Goons is played against you, since it lets you draw back up to 6 cards.
By killing off the Coppers, my Watchtower leaves me with a much more streamlined deck than my opponent. This, in turn, means I play my Goons more often and earn more VP chips, so I can stall the game as long as I like. My opponent is forced to rush an ending before my Goons VP chips get out of control, but it’s already too late.