It's been a few years since I looked at the official server setup (I immediately switched to bitwarden_rs, this project), so maybe this isn't as true today:

Mostly it's that the official setup is the same thing that runs their free/paid hosted service. By design, it is quite split apart into separate areas, presumably so they can scale independently of each other: https://github.com/bitwarden/server/blob/master/SETUP.md

But most people are going to want to host self-hosted instances for themselves, their families, or maybe a small company. The official server might be the best route if you're deploying to thousand of users, but that won't be most people.

Another benefit is that this flips all of the "paid" feature flags to true. If you self-host Bitwarden's official setup you still have to pay a subscription to use paid features.