Spaying a female will prevent Pyometria, which is an infection of the uterus that can kill the cat and any potential kittens should she get pregnant.

My munchkin cat Shorty had pyometria. The breeder refused to emergency spay her because she wanted her to have kittens. Shorty spent a week in the hospital with an I.V. drip running. She came home (I was fostering her at the time) and a week later the Pyometria came back. My vet did an emergency spay to save her life. The breeder (this is the one who kept Shorty in a cage the first year of her life till she came to live with me) had no use for her anymore because she couldn't produce babies, so she gave her to me. Hey, Shorty is the best thing that ever happened to me. She is now a happy little Munchkin with a tail that wags constantly, instead of this scared little creature who had no human contact and hid and cowered in corners.

Breeders like that really make me mad.