I don’t know, but here’s what I think I found out.
But first–My question is why do you care? Is there something you’re trying to do? But maybe if you’re trying to create a plugin that has user options, this is how to to do it. It’s not used in core since it’s a hook especially for plugins to be able to have their own user options.
Looks like it’s used in the Chat plugin like here:
Looks like if your plugin has user options then you use this to establish that it’s a user option with the UserUpdater.