They take cracked games and compress their size down so that shipmates with low bandwidth can download them easier.
I only just noticed that they don’t work if you are offline so they are worthless. It means it’s connecting to a server that could disappear at any moment and make your downloads useless.
That is not true. I use Fitgirl repacks regularly and you can definitely install them while offline. You probably just accidentally checked (or didn’t uncheck as I believe they are checked by default) the option to download additional files (see the three examples in the screenshot below).