I am not a breeder yet, but I plan on it in the future. Hopefully by next year I will have my foundation bitch, and in about 4 years I will be planning a litter.
My reason for wanting to breed is really simple: Improvement of the breed.
Border Collies, as a breed, have a lot of room for improvement, and they especially need help along the lines of versatility. (Example: It is quite rare that you will see Border Collies from working lines with structure good enough to finish their conformation championship.) A lot of BC breeders seem to have gotten so caught up in the "working dog vs. show dog" politics that they have forgotten all about their original goal: "the good of the breed".
My goal in breeding will be to improve the structure of the working BC, and still preserve the hard, "original" BC temperament and working ability.
I've chosen to breed Border Collies because I love the breed. They are, to me, everything a 'good dog' should be.