Because they are usually so abundant, spiders are among the most important predators in many ecosystems. All spiders are predators, and because the main item in most spider diets is insects, humans benefit enormously from spiders.
Spiders are important in controlling insect pests in most agricultural crops.
By controlling insect population sizes, they keep most potential insect populations too small to be economically damaging, most of the time. For this reason, the importance of spiders to agriculture has generally been underestimated. Spiders also eat many insects that bother humans, such as mosquitoes and cockroaches. Since a single spider may eat many mosquitoes in one day, over the course of its lifetime a spider may prevent hundreds of mosquitoes from surviving to produce even more mosquito offspring. Additionally, spiders are an important food source for birds, lizards, wasps and other animals.