Just like there is good bacteria and bad bacteria in the human body, there are good pests and bad pests in gardens. In general, earthworms are your garden's best friend. They dig through the soil providing holes for water to sink through which is exactly what you want. Brown snails, however, are garden killers. Luckily, there are ways to help your garden while repelling snails at the same time, like spreading eggshells and coffee grounds on your soil! The snails may hate you, but your plants will thank you.