: Who needs slaves when you've got robots?
I think you're forgetting that pfhor are not just slave owners, but slave traders as well, selling low-technology species to higher tech civilizations if Leela's terminal of dubious historical accuracy in M1 is to be believed.
The pfhor also need to assert power over worlds to colonize with their grossly increasing population. It's not likely admiral tfear's job was to find life-bearing planets and say, "Hey, let's be friends. Mind if some of our species park here and share your planet? We need some resources, pretty please."
Machines also require resources to be produced, and they could pretty much literally gut entire planets to harvest machinery to serve the needs of every living pfhor with how many of the three-eyed things there must be. And what do they do, make robots whose job it is to mine for robot metals? The pfhor probably have a "TAKE TAKE TAKE EVERYTHING" mentality, and and adding readily-available living species to their robot labor force speeds progress up a little while keeping what could have been potential rivals subjugated under their rule.
