The files you download could just as easily be unzipped and stored as one big avi(XviD/DivX)/bin-cue(VCD/SVCD)/img(DVDR)/iso(Data) file, but it is stored that way for the early days of usenet (news) sites where people often used dialup which is non-resumable. If the download failed, say due to a line drop, they would have had to start a 700MB+ download again, with them zipped they could lose only upto 15,000,000 bytes. For CD it is normally 15,000,000 byte chunks, and DVD it is now 50,000,000 byte chunks. WinRAR also ensures CRC checking still, so even though BitTorrent checks the hash, it allows you to verify the original torrent didn't have any errors in it, you can even tripple check this with the SFV files included with nearly all RARed releases. On movie releases the RAR files are not allowed to be compressed (so people can check quality not only with the common sample, but by dragging a single RAR onto VideoLAN, works with MPEG files (VCD/SVCD/DVDR). However with new game releases containing loads of dummy data (for copy protection, which is very easily compressed as it is not complex data) compressing of files is accepted (won't get nuked) if the file size is reduced significantly, for example a 4GB game containing 2GB of dummy data could be compressed as it would be a waste of time and bandwith to download so much uncompressed junk data, and on newsgroups bandwith is far more impornatnt than P2P as users normally have to pay per GB they download.