The easiest answer to that is that he's keeping his species pure. Many go further than this and keep their strains this way. For example... 1/2 black red guppies in one tank. Yellow snakeskin in another. That sort of idea.

The biggest reason is as Lisa said. You will want to be able to know what is in your tank when you are done. I breed mostly goodeids and many of the female goodeids from different species look much alike. If I didn't keep them all separated, I would run the risk of having a crossbreed in a future generation. That is definitely something that I do not want. Even some of the common aquarium varieties of the most common livebearers like guppies are worth separating if you are a serious breeder and not just a person with a tank full of fish that sometimes breed. If that is all that you want to end up with, mixing hurts nothing at all. In my larger community tanks, I don't worry much who breeds with who because those resulting fish will never leave my tanks. It is an entirely different situation to my serious breeding efforts.