They don't always, though. The kvaldir, for instance. We were introduced to them back in Wrath where they were this mysterious race of vrykul who were deadly threats to all coastal areas. Then besides a cameo in Vashj'ir where they were shown to have warred against the naga, and nearly winning, in the past that was it. Until in Legion we finally find out where they come from, who created them, and what their objective is.