These primitive civs aren't the underground feature ones. They're overland civs, so they'll make villages and spread out and trade and war and whatnot with everyone else. Unlike the main races, however, they're very limited technologically. They can only use copper, gold, or silver items and have a very limited range of armor and weapons that they can use. Not that this stops some of them from being very deadly (snakemen), but it generally means that they lose if they get into a war with any of the major races.
Gameplay-wise, it adds some more flavor to the world. Most of them will come trade with you, a few will try to steal items from you, and you might end up at war with them. I'm sure they'll make things much more interesting in Adventure mode than they do in Fortress mode, especially since Toady is expanding on that.
I created the Primitive civs as a standalone mod back in 40d and quite a lot of people liked it, so I figured I'd fold them into Civilization Forge this time. I'll also release a vanilla version of them as a stand-alone mod.