Invertebrates can be undervalued as inhabitants of your reef, having biodiversity has worked well for nature. Every organism has a purpose, and can help stabilize your tank. Some snails/mollusks eat specific types of algae, others focus on sand, glass, rock, detritus, etc… Not many species that aquarium keepers trade are harmful or predatory, and for the most part all will enhance your marine keeping experience. Some species are best housed in pairs, nobody likes to be alone. Animals will live longer if they are content with their tankmates/surroundings and aren’t subjected to stress.
These harmless animals should be in every reef tank!