Sea urchin are voracious algae eaters. Unfortunately they also tend to try to rearrange the rockwork in your reef tank, and they grow big, and strong enough to cause some real damage.
Sea urchin (Strongylocentrotus sp.) on the glass of a reef aquarium.
Sea urchins love to pick stuff up and carry it around. Unfortunately they do try to pick up your corals, small animals such as snails, etc. that do not like the harassment.
Sea urchin carrying stuff.
Sea urchins do eat a lot of algae, but some species like to eat other things, so one must be careful when one picks a species of sea urchin for a reef tank. I like to have sea urchins in my reef tank because they eat so much algae, but then I tend to swear at them when they start knocking over corals and rocks.
Most sea animals love to eat the innards of sea urchins, they just can’t get to it because of the spines. Smarter creatures, mainly sea otters, can crack open a sea urchin and get to the food inside.
Sea urchins are also edible to humans and often served in sushi restaurants as uni.
Sea urchins are also used to study embryonic development.
Further Readings:
The Sea urchin article on Wikipedia
Sea Urchins on MarinelifePhotography.com
Pacific Urchin Harvesters Association (PUHA)
Raising Tastier Sea Urchins For Foodies And The Environment
Sea Urchin Embryo Cell Division
There is a blog devoted entirely to Echinoderms.