The pnpbios bus provides a different way to attach devices than the real buses (e.g., isa(4)). The reason to use pnpbios instead of the real bus is that some machines have weird routings of, e.g., interrupts. The exact information about these things can be found in the BIOS, which is why the pnpbios bus has a better chance of succeeding with getting it right.
Many older BIOS implementations do not support PnP, and some newer ones do not follow the standards.