they do, i guess trample. but for the most part, a duck will eat the pests, and your
compost pile more or less.
people often use ducks instead of chickens because of this. chickens will eat everything, ducks won't though. at least that's my understanding of them, i don't have first hand knowledge of them in gardens specifically, only what i've read, and watched on the interweb.
especially once your garden is big.
even geese can be trusted with certain crops. like potatoes.