They mean that you can specify dependencies. So start some service Y only after X has been started. I use this extensively in my home automation services. Can't start other services if the MQTT-Server hasn't started and that isn't done until the network is initialized.

Sure, but doesn't that also make them slower?

Hardly. I'm an OpenRC user. Booting OpenRC vs SystemD, desktop, server, VM. Near zero difference in boot time, neither side was a clear winner, nor always faster.

My favorite part is OpenRC still likes the old ways, has script extensions for new ways, and runs the same path every time.

That's just anecdotal. If it's a technical fact that SystemD does parallel upstarts where OpenRC for example does not, that just means you don't have a system where parallel upstart makes a difference.