I have really slow home Internet (would take a day of download time), but a fast connection at work, so I installed the game temporarily on my work PC via the Steam client intending to backup at work, then restore at home. However, Steam’s backup/restore method is a bit clunky. Looks like a much faster to copy games from one PC to another is to simply copy the game folder from it’s location on the PC where it is installed plus it’s associated appmanifest file to the destination PC. That way the Steam client on the destination PC already thinks the game is installed. You of course need to own the game because you still access through the Steam client.
The backup procedure allows you to compress and chop into sizes for CD, DVD, etc, but the space savings is only about 20%. For instance, the game folder for JC3 is a little over 50 gb and the compressed backup files total a little over 40 gb