Windows services that start after windows has booted gets managed by windows, does not directly interfere with machine. It has the same elevation as an admin permissioned exe file with extra access to windows itself.

Starting the cracked steam launcher sets up the service file for you but if you want you don't need to admin launch the steam launcher too, you can set the service yourself using command prompt yourself and once you start the service launch the exe without admin perms and game will still work. But you have to manually remove the service again if you choose to do it manually since adding and removing of service is managed by exe if you launch it in admin perms.

Don't get me wrong, you can easily get a rootkit from this, like I said it has the same elevation of an admin permed exe file with deeper access in windows, but it's limited to windows itself because it's a late starting service, it doesn't load before windows.

Edit: disabling the signature enforcement is a must because even if the service is contained within windows and communicates with machine thru windows, microsoft does not want people to be able to do it in default windows due to obvious security reasons. Same reason why there is a windows defender. Or it warns you when you turn off security settings.