It provides an alternative UI environment built and optimized for gaming. It has a separate windows manager, a complete ui, and a set of menus to simplify customization of whatever is needed for gaming and power saving.
And quick access to steam store.
It is extremely convenient if you like a console-like experience, but, if you are a tinker gamer, it has anyway a lot of nice additional features.
It is inconvenient as general purpose desktop os, because on update you basically lose packages not installed as flatpack