It's not how you would normally specify it, but it is an option: the normal usage for it in systemd is enabling and disabling which services start on boot: an enabled service usually gets set up as a dependency of the multi-user target which is what systemd starts on boot. (And you can get a list of dependencies from systemd if you want to debug anything: the WantedBy stuff just turns into some symbolic links in the filesystem if you want to inspect things manually)

I don't know why it's being used in that way for these containers. It'd be easier to just add a Wants line on Caddy.