Because RAID is a High Availability technology.

It's not a backup. It doesn't protect against mistakes, bugs, or the server failing in some other way. What it's good for is for ensuring that work keeps happening if a disk fails. And disk failures happen to be fairly frequent, since spinning rust is a rather delicate technology.

If somebody has to connect to the machine and fiddle with it by hand it means that the system has been down, possibly for hours, when that was the exact thing you were trying to prevent by setting up RAID on it.

You don't have to modify it after a failure. You can set it right now to explicitly say "when I end up in a degraded state, I want to boot anyway".

> What it's good for is for ensuring that work keeps happening if a disk fails.

Different services have different priorities. I want my work servers to keep running, but my home server under the desk to stop until I replace the failed drive. Btrfs gives you a choice.

No, because I want to be elsewhere on a Sunday evening than in front of a computer modifying fstab to bring a production server back online.

Unless you want to reboot the server this shouldn't affect you in any way.

Unless you get a power outage or some other event that causes a reboot (e.g. STONITH)