Should I use the UI as a game space?

In the various tutorial exercises I've been following I've been seeing both no UI use for the objects in the game and exclusive use of the UI for objects in the game. so my question is for the objects in the actual game that the players play, is it better to design them outside of the UI and exclusively use the UI for the game's GUI or should the entire game be designed as part of the UI, or is there some middle ground I've yet to see?