These days, most games that require internet connection like MMORPGs or MMOFPSs are the most popular. Most single-player games are often overlooked. As soon as I get myself a gaming PC, I already know some good online multiplayer games I can play but what if my internet connection goes down? I would need some good offline games to play to pass the time.
And in theory, you can set your Steam to Offline, then play the offline features of those games. You need it online beforehand to install them, though.