They’re fully featured Systemd services with root privileges. Due to the AppArmor MAC and other environment quirks, root won’t be able to touch the regular desktop users Pulseaudio daemon/session. Running the command via the shell runs it as your current user with the normal user environment supporting it.
Assuming you have a desktop session running and just want to have the app open automatically on login, you’d want either:
User Daemons (Experimental and unlikely to be stable any time soon, it currently needs enabling per snapd installation).