But only for devices already on that tailnet.

This allows you to provide information to an arbitrary person (a friend/coworker/etc) to let them access the thing without them having to jump through all the extra hoops of joining your tailnet/them joining yours/adding a VPN/etc.

With Tailscale at least, you can pretty easily share a node with someone else. If your target audience are solo developers or hobbyists, making it even easier to share access is surely nice; from the perspective of someone in charge of making sure our company IT is balancing security and ease of networking, the literal last thing I want is making it easier to grant someone access.

There are policies defining who can talk to what; they are deployed from a GitHub repository with defined rules on who can modify them and who has to review them; there are zero scenarios where I want an alternative way of granting access to any device or service under our control.

but what exactly is the use case? I was responding to the nat traversal topic..

If I wanted to share something internal with a friend I would use ngrok or any of the million alternatives.

Anyway, this is exactly why my top-level comment says that this project needs a "versus" page in the docs.