IThat's a good answer, although not entirely accurate. Everyone in the team put a lot of emphasis into making it possible to go anywhere, whenever you want (at your own risk, of course

), and still be able to tie things together (quests, for example, if you decide to bolt to the Strip right out of Goodsprings).
The actual purpose of any such barrier is to prevent other more egregious oddities from happening. As you know, creatures, NPCs, and Companions, can try to follow you anywhere... and I mean,
anywhere. There are some spots where you, as the Player Character, can easily get to (jump, strafe the side of a steep cliff, etc.) but where creatures, NPCs, or Companions, just can't make it. In cases like this, it's an usual trade-off solution to bound a region with rocks, cliffs, and, yeah, sometimes invisible barriers when there's no other better option.
O.