It tricks Windows into treating the repack like a physical disc, bypassing certain file-path-length limitations and permission errors.
Rarely, a specific FitGirl repack version of a game has a bug that triggers Error 14 for everyone (check the comments on 1337x or FitGirl's site). If you have: unarcdll error code 14 fitgirl
Verifies your downloaded files aren't corrupted before installing. It tricks Windows into treating the repack like