> What would be the legitimate use?

Caching/storing GUI state and user configuration.

You (the server) doesn't need to know about LocalStorage contents. You can read and write from it via JS without ever sending that data somewhere. It actually improves privacy if done this way, because then the user owns the data and you just act upon it.

Unfortunately Apple thinks otherwise, they clear LocalStorage in Safari after a while by default.

See my comment above, localStorage can not be shared between domains as cookies can. So it's a lot better for privacy. With localStorage you can only track multiple visits of a user on the same site, not across sites.