MBR is the master boot record, which consists of the master boot record, the primary partition table and the signature "55AA".

PBR is also called DBR, which means disk partition boot record. It has two parts: DOS boot program and BPB.

I think the key is where the boot record is. I want to boot from MBR first and then to DBR, and then DBR loads system files into memory.

As for why, I think if MBR is the root, then DBR is the stem. The guidance of the system is like the transfer of nutrients. Without any one of them, it cannot be fundamentally transported.