So when domains go down or are taken down, access to the services don't go with them
Same reason you need Nostr when "twitter already exists"
But what advantages does this have over Tor or I2p hidden services?
Cats should be skinned many ways. Different approaches are better or worse for different contexts. Also, surprising net-new discoveries are made beyond the frontier when you re-solve a problem with a new stack. Sometimes some property of the stack, in connection with the "already solved problem", reveals novel new territory. (Look at "the microblog UX" getting "re-solved" by Nostr)
Cats should probably not be skinned