It's about making a world that appears to have a life without you: if you can't see where a thing ends, as far as you know, it doesn't.
Even if you choose not to go down that particular rabbit hole, the game benefits just from it being there. You see a town with a couple of questlines in it, and you maybe talk to an NPC or two to get some more info. But it's the mere presence of that content that makes the world so engaging, whether or not you actually choose to engage with it. I'm never going to play The Witcher 3 for 400 hours, or 200, or whatever they've said the upward limit is, just like I'm never going to play Fallout 4 for 400 hours. I've been thinking of this in light of The Witcher 3, which I revisited a little bit in preparation for the glut of open world games we'll be getting in the coming months, starting with M etal Gear Solid 5: The Phantom Pain and Mad Max on tuesday.