Why spay and neuter rabbits?
Altered rabbits are healthier and live longer than unaltered rabbits. The risk of reproductive cancers (ovarian, uterine, mammarian) for an unspayed female rabbit is virtually eliminated by spaying your female rabbit. Your neutered male rabbit will live longer as well, given that he won't be tempted to fight with other animals (rabbits, cats, etc.) due to his sexual aggression.
Altered rabbits make better companions. They are calmer, more loving, and dependable once the undeniable urge to mate has been removed. In addition, rabbits are less prone to destructive (chewing, digging) and aggressive (biting, lunging, circling, growling) behavior after surgery.
Avoidance of obnoxious behavior. Unneutered male rabbits spray, and both males and females are much easier to litter train, and much more reliably trained, after they have been altered.