Rar är till för att dela upp stora filer i mindre paket. Om det blir fel på någon fil så behöver du bara ladda om den skadade delen av filen isället för att ladda ner hela klabbet en gång till. Hoppas att någon fattar vad jag menar.;)
Det används väldigt mycket på tex PirateBay, filmer man laddar ner kommer i kanske 50st 14MB-bitar (varför just denna storklek?).
Kollade lite på WikiPedia:
In computing, RAR is a proprietary file format for data compression and archiving, developed by Eugene Roshal (hence the name RAR: Roshal ARchive).
The filename extension used by RAR is .rar. If a RAR-archive is broken into many smaller files (a "multi-volume archive"), as is often the case when it was distributed on Usenet, then those smaller files carry the extensions .rar, .r00, .r01, .r02 etc. and most unrar programs reconstruct the whole archive when provided with the .rar file.
Roshal created the RAR file format and developed programs for packing and unpacking RAR files, originally for DOS, which were later ported to other platforms. The main Windows version of the archiver, known as WinRAR, is distributed as trialware, requiring payment after 40 days (although it can still be used after this period, albeit with nags); shareware versions of this program are also available for Linux, Mac OS X, DOS, OS/2, and FreeBSD, though they are all called simply "RAR".