Debian tor.service explained?

Thanks for the thorough reply @jeremiah - I’m looking for the reason that the tor process is installed and operating.
How is it the otherwise quiet system using it and why is the service running?
I know systemctl/service as well as tor and its uses, but I’m not sure about the how/why of a tor process on boot like this.
Another way of saying it: Why is the system running and using a tor process outside of a user initiated action?