Well I was thinking of that too. They would probably use them for work, wool, possibly meat (though I was thinking they would limit this to monsters that had no other use), transportation (I'd imagine giant flying monsters would be pretty popular for far distance travel and trade). Some may even be kept purely as pets or for dealing with vermin. Oh, and if some monsters could actually summon rain like some poke'mon can, I'd imagine farmers would be using those a lot, particularly in more desolate climates. They could also possibly use electricity-generating monsters to well produce electricity, assuming they had the technology to utilize it. They may also use fire-types for steam power, or just to heat water. They could also use them to start campfires too I guess.

Oh, another kinda comical thing I just thought up: waste disposal. I mean there are poke'mon that can produce acid, and I think some can eat things that are normally inedible. I imagine some could eat food waste.