I think the most probable reason for this instruction is for calculating parity bits. This would need to be done fast so it makes sense that there would be a CPU instruction to do most of the work.
Parity is much easier than counting. It's so easy that you get it for free in the x86 flags register. (… and because the 8008 was designed to run a terminal.)
Each iteration from the 8080 through x64 have a parity bit in the flags register for backwards compatibility with the previous generation. The 8008 was a microprocessor implementation of the Datapoint 2200 architecture.
Early protocols didn’t have error correction in the lower layers. The parity flag was equivalent to a CRC instruction nowadays. Presumably if parity was incorrect, that would mean the byte was transmitted incorrectly.