rar

The fixes are packed using rar. Why? Why not just use the windows zip thing that everybody has?

I guess that is not as important as what rar program you recommend to unpack these files?

Thanks

7-zip: it is free, and in addition it can unpack the installer as well (should you ever need it).
WinRAR will work as well.

To the reason why the archives are not packed using the .zip format I can only guess; but the fact that .rar achieves much better compression rates could have something to do with it. (Btw, some of the fixes do come in .zip format.)

No particular reason except convenience. Winrar has easier shell integration than 7-zip (which compresses *slightly* better than rar actually), requiring less clicks.