The main reason the land exists outside of the map borders is so the land inside the borders looks right. In other words, the western slope of a mountain in the game needs an eastern slope too, it's just outside of where you can go. Any of you who have played Simcity and terrafromed or tried to create land masses with the other TES editors will know what I am talking about.
As for paths and structures and such, it is probably a placeholder for any future DLC that comes out.
Also, we don't know the plans for TES6, but maybe they already built the map for it and it's in this game. This way when they build the content for the next game, the map looks like it belongs.