They bring back that sense that there's more to the world than what's actually shown in-game that the 2D games had. The 2D games had this naturally; we didn't take the small 2D maps literally, we saw them as an abstract representation of something far bigger.

With 3D graphics however, we take the in-game map literally. The only way to bring back that sense that's more to the world what we can access is by literally having inaccessible areas in the game.