par files are totally kickass.. you don't need to download them, but it is often a good idea. For each par file you have, you can complete 1 corrupt file in the archive (assuming the size is the same).
So, if you have 100 1mb files and 2 are missing, any two par files could be used to regenerate the 2 that are missing. Last time I download from newsgroups, I used a program called 'Winpar' though I dunno if that's still the standard.
I once downloaded a 4-cd set of isos and I was missing the last 20 or so 15mb archives. But, the guy that uploaded it put up 20 par archives.. it was just enough for me to recreate the archive completely.