Sorry, I guess my question wasn't "why does the tree need a root process" but "what is the critical feature that makes it worth using these tools instead of the dumbest possible alternative?"

So the dumb response to your questions is if I have a list of services: "what else do I launch?" everything, "what order do I launch it in?" launch it all at the same time, "what do I do if a service I started stops on its own?" restart it. I typoed, but I really meant my first sentence verbatim: suppose init is a bash script that just does:

    (while true; do service1 &> log1.txt ; sleep 5; done) &
    (while true; do service2 &> log1.txt ; sleep 5; done) &
    (while true; do service3 &> log1.txt ; sleep 5; done) &

On a desktop I'm not starting/stopping systemd services ever. Every process I launch is managed by the window manager. On servers it's mostly the same: start everything at boot, then the server does its thing until shutdown. On more traditional mutable SMB shared servers I can imagine sysadmins going in and stopping/starting/adding new systems, but I feel like that use case is gradually growing less important.

It's largely touted that systemd's killer feature is that it represents services as a graph. But why is it important that it operates as a graph? Why is ordering startup important? Or does that not matter, and the real reason systemd matters is that it makes packaging easier (as you said, and as I said) and it has lots of built in conveniences? I have my guesses, but I'd like outside opinions.