PAR files are created by DVD SP for playing back Mpeg2 files. These files can be deleted, but will be recreated when you open the project again.
If you open a project that has the PAR files deleted, you will get a dialog window stating that it is parsing files as it is opening the project, this parsing causes a delay in the opening of the project.
Mpeg 2 playback can't take place without its associated PAR file, however deleting PAR files will not stop a project from working, due to DVD SP's rebuilding them each time a project is opened.