There are many beneficial reasons to spay or neuter your cat (or dog). Many of them are health related. Unless you are an educated breeder and plan on breeding that particular animal, spaying or neutering your pet is the best thing you can do.
In females, spaying your cat (or dog) before her first heat cycle can reduce, if not eliminate mammary gland tumors, ovarian cancer and uterine infections and cancers. Uterine infections and cancers are fatal in 90 percent of cats. A spayed female will have less desire to roam and therefore be a better companion to you and your family. Also a spayed female will not go into heat, which can be very nerve-wracking for her owners and her owners neighbors. Intact females will yowl loudly and some will urinate all over your house during heat cycles.
For males the health benefits include the elimination of the risk of testicular cancer, decreased risk of prostate disease, decreased aggressive behavior and less likelihood to roam, which reduces the possibility of him being hit by a car or getting into fights with neighborhood strays. Intact males will spray his surroundings, such as your home, with a very strong smelling urine. Your neutered male will be a better companion pet, focusing his attentions on his family, not roaming around looking for a mate.
Many people think that neutering or spaying their cats will make them fat, but proper diet control and keeping your pet active by playing with him or her can eliminate this problem. Keep an open line of communication with your vet and he or she can help you keep the amount and type of food you are feeding your pet appropriate for your pets age and any health conditions.
One last note, every year there are millions of pets from unplanned litters that suffer as strays or are euthanized. Check with the Humane Society or a vet and there may be a plan in place to help deflect some of the cost of the spay or neuter surgery.