Unit files are simple to write, and make incorporating new custom processes even simpler (does it log to stdout? It'll log to journald and then syslog perfectly fine).

Process management is built in and sophisticated (restarts, timeouts, conditions, limits etc.).

The dependency system is amazing for ensuring your system fails sensibly: I can link mountpoints to processes, and easily guarantee that a mail daemon only runs when its network mount is ready, and then make it also shutdown if that mount goes away for any reason. This is huge for self-documenting infrastructure - you can ensure that even manual intervention in the system won't leave it in a weird state, and the why is all right there.