"Useless" is a strange way to put it, since I don't see how background material would need to be useful, and not just interesting to the intended audience.
And of course there aren't multiple people with the same backstory. That's what you call ludonarrative dissonance. The same way in Slay the Spire your character isn't literally using a deck to fight their way up, even though a lot of in-game events involve getting or discarding cards. Or in DD2 you shouldn't imagine there is a straight road cutting through the world with just a couple of bifurcations here and there. Or how the way loot drops work in most games.
I'd argue that the individualization is a bonus, but the narrative in DD1 is exactly trying to make you treat your "heroes" like expendable minions, letting them die or dispensing them when they are not useful anymore. It starts by selling you an idea of redemption, but then pushes you into really becoming the Ancestor's "heir" in the worst way possible. The cosmic horror aspect takes precedence to making a character piece on all hero classes.