When dealing with a lot of different servers sometimes a new post can be damaged or transferred in an incorrect way. By using the PAR files you can repair these damaged new posts. This way you do not need to download everything once again. A good program to handle PAR files is QuickPar.
A list of good programs for dealing with PAR files:
Windows: QuickPar
Mac OSX: MACParDelux
Linux: PyPar2