I'm not trying to be "smart" or anything, though this is going to sound that way. The difference is, we want purebred birds of a certain color or lineage rather than just whatever we can easily get. You can still get common layers at the feed store here for $3.50 a chick and let them breed each other just like our grandparents did.
The difference is some people raise birds for the joy of it, some people do it for food, some people do it both, and some do it for pure profit.
In the long run, it is short lived though- you'll have to keep bringing in new things you can charge high prices for to maintain the boat loads of cash you make when they are first released. If I am not wrong, the more importing you do, the cheaper it gets.