Weird sentence when translated literally, yes. But imagine a context when you were in a group arriving at a house party, and you wanted to say that it was you (pl) at the door. "It's us!" is completely standard English, and I presume that "somos nosotros" does the same job in Spanish.
While this is a weird sentence when translated word-for-word in English, we have to keep in mind that not every phrase translates EXACTLY into an English phrase. "Somos nosotros" makes sense in Spanish even if each exact word doesn't exactly translate into English in the same order.