Cracks are usually released separately from the uncracked game files. Repackers take those cracks and package them with the correct version of the game, compress the files and add an installer. Then they upload them to the more mainstream public trackers.

Repacks have several benefits. They tend to be easier to setup and usually more reliable. They download faster and use less data because they are compressed. They are also sometimes packaged with extras like soundtracks, mods, etc.

Fitgirl repacks are known to be more compressed, so the files are a little smaller but take a fair amount longer to install.