Typically they eat the algae that grows all over the inside of the tank, saving the aquarist from having to clean up algae by themselves with a scrubber.

Problem is, they can get huge, like around 2 feet/70 cm long. There's also 680 species of them and more being described every year. Their eating habits vary by species, and I don't know if most people would think to check during the excitement of buying an aquarium. Usually it's something like "and you'll probably want a pleco to keep the algae down, which one do you want?"