Are you talking why for the user, or why it was developed? The main reason it exists is that System 76 like the Gnome desktop, but didn't like stuff Gnome was doing, so they decided to make their own version from scratch in Rust. For a user, I don't think there's any real compelling reason to use it, especially not right now, unless you love Rust, or have the same feelings about Gnome that S76 did.