So and so quits and their docs vanish!
Let's say you have a service where every user is an account, and those accounts might have data associated with them in a big catch-all database. They might be (redacted) or (redacted) or even (redacted) which was imported from some other format. People interact with them through your service and all is well.
Then, later, you grow it. You start letting entire groups of people work on these same items. Maybe you organize it by some kind of identifier, like an X.400 OU or a domain name. You start pitching it as a service for an entire organizational unit (that's what an OU is, after all) and people start using it that way.
Then, one day, someone leaves, and their account is deleted. A couple of days after that, someone else who's still at the organization freaks out because all of the items which were owned by that former employee have vanished.
You made the mistake of having the individual humans own the items, even though it was really owned by the organization itself. They did the work for the company, and the company owned it. But when they left, your auto-deleter kicked in and toasted their data. Oops.
Some systems do not automatically scale to handle organizations. Don't ask me why I know about this.