Algae munching, rock scraping urchins.
Be advised, I have seen some urchins, coldwater, temperate, tropicals, etc. appear to release poisons/toxins into the water column, usually when they expire, and some of these guys are quite venomous. I would probably not trust any large urchin(s) in the same tank with a prized collection of fishes but that's just me. On the friendly side, my favorites are the gorgeous red slate urchins Heterocentrotus mammillatus, also the clunky old pencil urchin Eucidaris tribuloides which I have collected a lot of here in Florida over the years.
Not sure anyone would ever want to ship in bags or keep in tanks the long spined Diadema variety; those guys are nasty!!! I have always avoided them while diving for good reason; those long needles are extra painful/easily broken/embedded and once broken off the little pieces can actually work/twist their way into your flesh until they disappear, sometimes for a year or so until they surface again elsewhere.