1 ) Preservation. Without another breeder like myself, the breed or color variety would be lesser known and/or have a greater population of poor quality examples.
2) Self sufficiency. I want to raise my own, eat my own, breed my own.
3) The love of it, of course.
I love certain breeds of chickens, and the best way to do that is to really get yourself into them. Have multiple males and females, learn their genetics, learn their behavior, their type, etc.
4) Similar to the preservation reason, but, endorsement. I want other people to get to know the breeds I love, and see why I love them so much, so they too can have such fascinating and useful breeds. Some people are looking for the ultimate free ranging, dual purpose, colorful, and rare bird, perhaps with colored eggs, well there they go. . . Or perhaps someone would love the ultimate "pet" chicken, who honestly also looks like a dinosaur, and is a walking piece of history.