It's SCENE common practice. The Scene being the collective of sub-scenes/warez "groups"/hackers/crackers, They're behind most of every pirated thing you can get off bittorrent. To RAR your release is a rule, to RAR twice is not unheard of.
First thing to know is that The Scene is not a fan of bittorrent; The things you find on bittorrent by scene groups and member are usually up there against their will. Every idiot in the world can torrent, and thus it acts as a giant spotlight onto the groups. It's like having a secret clubhouse but certain members decide to steal off and let everyone know what's going on inside.
Bittorrent makes null many reasons the scene uses multiple RAR format. Other than the mighty compression possible with low compressed data, it allows Hash checks for corrupted archives and selective downloading in case one or more archives are bad. Bittorrent already does both of these things. It's safe to assume the reason most torrents are rared is more because an unwillingness to alter the source files from the scene, from laziness or as a form of tribute, than because of any positive effects of a RAR archive.
