That technically works, although it doesn't handle dependencies, registry entries, and more important it doesn't contain the license to play the game. That means even if you copy it to another computer with steam, it won't work unless you sign in and acquire the license from steam (for that computer), and that might involve doing a full update or re-download of the game, i'm not entirely sure of the requirements so i'll assume the worst.

You can use the back up feature or just copy the directory. If you do the latter, then when you run the game it will download what ever it needs. If it doesn't, then you just verify the install and then it will download what it needs. But in this case what you want to do is copy the directory then if the game ever updates, you copy the old build back over the new.

Alternatively, you can use an open-source program called "Steam backup tool". It uses 7z compression, which creates smaller files overall. I found restoring to be even more practical than Steam and you don't even need Steam to restore your games (in fact, you must close Steam client when backing up and restoring games). It backups the game as-is, meaning not just original files, but mods too! If the game is without DRM, you can just extract the files from the 7z file using 7-zip and play.

Finding this thread again was a pain! Of course there's more on the bottom shelf... Sudoku and something else.