if a character has no propouse then why even give it a name and an unique character model?
Because those people should exist in any sort of realistic world?
The raiders have no reason to talk to you, and no reason to want to talk to you, but their bosses would still be fairly unique people, for being able to gather to many raiders under them. Should the raider bosses just be unnamed trash mobs like everyone else? No, that's why they give them set models, arms/armors, names, and terminals/notes detailing a bit about them/their rivalries with other raider gangs and what not. It doesn't mater if they are tied to a quest or not, they should be there, so they are.
The entire idea behind "rule of conservation of detail" is retarded in the modern gaming industry. This isn't 1995 anymore where budgets are so small you could buy a cup of coffee and run out of cash. You might as well ask "why bother making such a massive open world, with hundreds of various, and entirely redundant, ways to attack a target, that 99% of people wont use, in an Assassins Creed game, when you can make more linear levels, and just put like 10 different ways to attack the target?"
Assassins creed, GTA, New Vegas, any of Bethesda's games, all of them have tons of unique items/locations/NPCs/whatever around their maps that the player is never sent to use, and doesn't play a part in any quest. Its there because its supposed to be there, because it would be there.
Personally, I wish Bethesda would go back to what they did in Morrowind, and give every NPC an unique name/model, but the level scailing system makes that difficult.