The offline installers literally are the files to install the game.

It's as close as we can get in this day to having the disc and installing from disc long after the publisher was bought out and absorbed so many times nobody truly knows exactly who owns the rights to the game anymore. As long as your disc (in this case, offline installer) was stored safely and is still readable you can install it on a compatible computer (and that's often the harder part is finding a compatible computer!)