Part of it certainly is the fun-factor -- if I haven't been somewhere for six months of realtime, but I remember it fondly, it's neat to go back there.
But, in AVWW, it's also important. When you die, you take on the persona of another character you'd interacted with. If everyone at your newer settlements dies (ouch), then you're back to people in the older settlements that you haven't seen in a while. Of course, you could just as well ask them to move, so it's not like you're tied only to the older places you've been at, but still.
A lot of the things about the benefits of settlements and about improving NPCs that you work with I don't want to get in to too much yet, because they are a bit speculative at this point. But for one example, there will be certain "legendary" items and weapons and spells that require a specific type of craftsman, and improving certain NPCs in their crafting skills over a long period of time can pay dividends in that regard.
Granted, this raises a whole lot of other questions about fast-travel, crafting, and so on and so forth. Some of those we have tentative plans for, other things we're waiting to solve when we see them in terms of actual implemented gameplay, and our "plans" are actually just a collection of ideas that seem promising, but which we don't yet know which will be the best from. AI War evolved like this, too, and I think it benefits the game -- anything that can be fully comprehended from day one can't be that original.
A lot of that sort of thing will shake out in the alpha and beta phases, and we'll definitely be accepting player ideas on how to keep things fresh and interesting, too. There's nothing for adding variety like throwing hundreds of fans at a project each with their own ideas.