Why are annelid worms helpful to humans?

While some annelids are more harmful to humans than helpful, some are truly helpful not only to humans, but to other organisms as well. Earthworms help aerate the soil and act as prey to support terrestrial food chains. Other worms such as ragworms are used as bait and in aquaculture. Leeches, which are blood-sucking annelids, were important once upon a time in medicinal use for blood letting, are no longer important for that purpose but are still an asset to their native habitats.