Sometimes updates require a game to be restarted. With a launcher it never needs to REstart, it will be started fresh afterwards anyway.

It’s also in general a good place to centralise account management (esp. if parts of the game would need to be reloaded if another account is logged in!)

Lastly, although this is more of a side-effect, but it gives a good place to shows recent news/posts of the company about their game to players, such as when there’ll be downtime, in a way that’s practically impossible for the players to miss.