Well, that's the best part -- we don't really know.
Just like with most of the OLD ONES in H.P. Lovecrafts stories -- we kinda don't know what they want or why they're there.
Generally yeah -- in his stories, these Eldritch Abominations are shown being worshiped by different beings (often enough human or human-like), and we're all hearing about them and their purpose from the worshipers' perspective. They're world destroyers, they need a sacrifice, all of that jazz. But we don't get a clear "why". Even if we're told it, it's from another perspective, and it's often hinted that things get misinterpreted from the OLD ONES messages (if they even give a message at all).
I think it's just part of the fantastic narrative when it comes to cosmic horror.
The fear of the unknown, the unexplored, and not having a clear idea why these monsters appear. We can make up ideas and find some kind of conclusion, but it's always hinted that we won't know the real truth...and that if we did, in these stories, it'd be a truth no human should hear.