Off the top of my head: * Some VGM formats (game music) are compressed with rar * Certain 90s game mods are compressed with rar * Lots of pirated/cracked games are compressed with rar

The main reason I've been told as to why this is seems to be the extensive error correction, which was a big deal with large game files back in the Dial-Up/ISDN days where a corrupted download was more common than now, and old habits die hard.