I can only imagine that should something that would be a safety issue be found you would not be able to drive home after a failure. I had a few minor issues that could have been rectified at the IVA with the tools and stuff I took along but my biggest issue was paperwork related, IIRC an incorrect engine number on the certificate of newness that required a replacement. I’d also recommend checking over the car after the PBC, mines was returned with the rear wheels having been replaced on the wrong sides - my photo of the car on the flatbed as it left for the PBC showed I had them on the right way.
I’d recommend taking as much paperwork as you’ve got on the car, some of which you need anyway but definitely a certificate of insurance, just in case you get stopped by the police as happened to me.
Steve