People have pretty well summed it up here. I don't really like big snails myself but I am a fan of hermit crabs. They are fun critters and can help you break down uneaten food and dead live stock. To me, a clean up crew both adds and does not add to your bioload. They help convert waste and uneaten food into exportable forms. So that necessitates greater export capacity adding to your bioload. However, they also slow down the build up of detritus that slowly releases phosphates and ammonia into your water, reducing your reliance on manual removal and the need for water changes, thus lightening your bioload as that is concerned.
I have never found anything but an urchin very good at algae removal and they don't really keep your glass clean but a clean up crew does have a useful niche.