You can delete the PRM but you may miss it the first time there's a repeat of the job and you have to setup all your profile and plot parameters again. It will save all the various setting in the GSPPlot program such as repeats, gaps, and the file names of any images that were ripped into the four process colors. Having the PRM file provides greater assurance that your print will look identical the second time to the first time.
The only valid reason to delete a PRM file is if it prints with a defect and you want a totally fresh outputting of the file. PRM files are automatically saved in the same folder as the PLT file.