Character backstory - Better Source DM or player?

[Morgaln]

For a lot of people, that's enough. For some people, it isn't because they are looking for a deeper level of immersion into the character, and may assume that all of the other players do as well, which is only rarely true. (IME). The desired level of immersion into the character varies by player.

But then we're back to Batcathat's point; that kind of player will want a reason why they are going into the meatgrinder in the first place.
Also, this might just be me, but the meatgrinder as described by TemporalTravles would kill my immersion quickly; why do new characters show up suddenly whenever one of us dies? Where are they coming from? Why do we trust them? How do they know we need a replacement? And if they get sent by whoever sent us, why didn't we take them with us right from the start?