It basically just switches over Apple app accounts, such as Music and Photos. Everything else---all third party apps and the home screen layout and BT devices---are exactly the same.

It's laughably bad, since Photos doesn't even support more than one iCloud photo library per TV, so switching just makes the Photos app not work.

Mainly it makes the Apple TV+ app understand your watching history (“up next”) better.

Apple has an API so that third party apps can tie profiles to appletv user ids (iCloud ids) but I haven’t seen a single 3P app that uses it. Apple could make it mandatory at some point I suppose but the UX for connecting the two in each app would likely be tedious.

Also there is no password protection with changing user ids so kids can accidentally screw up your watch history / preferences. And without any passwords you don’t want to connect photos or anything like that to the tv.

It’s pretty much a botch, just like the cases described in this article.