In the game tab you can set the player prefab and the GUI prefab you want to use for the game.
WHY USING IT
The playerStart has some advantage among the previous system:
To make the game level playable you haven't to insert complex player and GUI prefabs into the scene (they can contain a lot of linked objects and a lot of logic), but a single and much simpler object.
PlayerStart is represented as a static character model that helps you to work well with proportions and dimensions when placing other objects.
Using playerStart Level files are a bit smaller.
It is always possible to ignore playerStart and directly insert player and GUI prefabs in the game level as previously.
Basing on your game you can decide to use only Player prefab and ignore the GUI, or you can configure the player prefab in order to contain a GUI itself.