You really can hatch a bunch of chicks out of just a few birds. This was our first year to breed and we are growing out more than 30 juveniles from only 2 breeding pens of 1 cock and 3 hens in each pen. We could have hatched more if we had wanted to and still had eggs to eat since we do have a small food-egg flock of birds that didn't make the breeding cut.
No matter how close to the Standard of Perfection the breeder birds are, chicken genetics are very complicated and they don't always mesh together in the way that you think they will (it isn't like the simple Mendelian pea plant genetics you probably learned in middle school). There are genes in chickens that only show up under certain conditions. You will still get birds that are not up to the SOP when you breed good birds. So if you allow flock breeding, there are infinitely more chances of genes coming together in the wrong way so that you get even more birds that are not up to standard.
It may be hard to understand right now, but after you get to know your birds and do your first hatches, you will see the difference that it makes when you make particular pairings of birds. And even then, sometimes things crop up out of the blue.
Example: From one of our breeding pens, we hatched a cockerel with droopy wings (wings look almost vertical instead of horizontal) and he has a side sprig sticking out of his comb. Neither the breeding cock or the hens he is with, have these issues, yet the dna mixed up together to make these defects in this particular offspring. In some of the other offspring from this particular breeding pen, we have some pullets that have tails that are looking ok, and others that have pinched tails. Some of them have the wrong tail angle for a Java, others have an ok tail angle. Some have good thick legs and some others have legs that aren't as big. Some of them are pretty heavy while others are not as heavy as they should be for their age and breed.
You just don't know how those ova and sperm are going to interact with each other. But believe me, once you do a hatch or two and see how they grow out, it will make more sense to you about how your choices affect the outcome of the offspring.