...Because startup systems like systemd and upstart are sometimes hard to tune in many situations, like MySQL data directory on a networked filesystem.
As rc.stupid does nothing other as running scripts (or anything that may be executed) you may find other use cases like simple clustering.
Small program to run from rcNG to start/stop user processes at bootup/shutdown. A few security checks are performed and the user's rcNG scripts are run as the UID of the owner.