> Isn't it pretty much just a repo/index for open-weight models?

No, its not just a repo/index (they also have training, inference hosting, and they develop/maintain a bunch of core AI infrastructure software), and even if it was just a repo/index, replacing a bug centralized repo/index that used by an large community isn’t trivial.

Not just a repo/index but they also offer reasonably-priced compute, with a free tier not to sniff at, on which to run the models.

Network effects. It's like a social network for LLM nerds as well as hosting and rating models, etc.