It means you don’t need to turn on proton for unsupported games the first time you start steam

(or alternatively you won’t be briefly confused why unsupported Windows games don’t launch on a fresh install)

It just enabled proton for all games by default instead of having to manually turn the option on. Just one less thing to have to remember or do now for all games with proton.

I think it just means Steam always offers proton for windows games by default. Current you have to set it to all games in the options or set compatibility manually for each game.