What's the use case?

Modifying the user member variables before they are saved to the database. Without a hook, you can't do any sort of validation, modification, etc. Say I want to examine the user's name and auto-update their email address based on that or auto create a signature for the user. Or generate a nickname based on their real name. The only field you can validate right now is the email address which has a dedicated hook.

https://gerrit.wikimedia.org/r/27266

Wooohooo!!! My hero!

The UserSaveOptions hook fulfills this functionality.

Nope it doesn't. Like I said in the original description, the UserSaveOptions hook is fired after the user object is saved and only allows you to modify user preferences. You can't for example change the user email address before the user object is saved.

That's not true at all. UserSaveOptions is fired before the options are saved on User.php:4397. Furthermore, the User object itself is passed as one of the parameters, so you can change the email address if you want as well.

You are wrong sir. The user object in the DB is updated on User.php:3080, then saveOptions() is called on User.php:3098 which in turn fires the UserSaveOptions hook. The email address, password, username, etc are already set and saved by the time the options hook fires.

You can modify the user object that is passed into the UserSaveOptions hook but then you will have to save it again yourself manually which doesn't make any sense since it's just been saved by the call to saveSettings().

The old SavePreferences hook used to fire before the user object was saved which allowed you to do things like automatically set a user's email address when you were using a SSO or PKI third party plugin to automatically log your users in. Perhaps there is some way to do this still that I'm overlooking but the UserSaveOptions hook is not it.