for some online games you may still need to reinstall the game & launcher(Steam, EGS, Rockstar, Battlenet, etc) even if you have properly backed up and restored all of your game files.
but, after a game is restored you should only have to point the launcher to the game directory and it would scan the folders and update/reinstall any missing files across the system. this would make it possible to avoid re-downloading all of the game files.
keep in mind though, like i mentioned earlier; any offline data stored on your system like certain saves and settings will be lost. this would need to be backed up and restored along with the game files themselves.