Spaying and neutering are two very important aspects of keeping your pet healthy. The medical terms for spay and neuter are respectively ovariohysterectomy and orchiectomy. Ovariohysterectomy is removal of the ovaries and orchiectomy is removal of the testicles.
There are many benefits of spaying/neutering your pet.
1. The feline stray population is at large, so by spaying/neutering your pet, especially if outdoor, or if you have other pets in your household which are not currently fixed, you will be doing your part in keeping the population down. Shelters are over-flooded with cats and kittens and there are many left on the streets to starve and die. Female cats are induced ovulators which means they will remain in heat until they are spayed (unlike dogs who only have a heat cycle twice a year), so they will keep on reproducing.
2. Spaying/neutering will help prevent unnecessary cancers. Female cats that are spayed before their first heat cycle, which is around 6-8 months, have a 0.25% chance of having mammary cancer. For every heat cycle that she goes through, she is at risk for mammary cancer. Male cats can also get prostate or testicular cancer.
3. Another non-cancerous disease which does have an effect on the internal organs is called pyometra. This also effects unspayed female cats (as well as dogs). Pyo means pus and metra refers to the uterus. There are two types of pyometra: open-cervix and closed-cervix. Open-cervix pyometra is when waste is allowed to leave the body and it's not closed in. This is the safest and easiest to treat. Closed-cervix pyometra is life threatening and an emergency. Pus and other waste is closed in the cervix and can not escape. When it can not escape it can cause the source to rupture and pus can escape into the abdomen and cause peritonitis and possible rapid death. The only way to treat a pyometra case is for the veterinarian to perform an emergency spay. A closed-cervix becomes more complicated since the veterinarian has to take extra caution to not let any of the pus leak into the abdominal cavity.
If your pet is not currently spayed or neutered because you are wanting to use him/her to breed, then please keep in mind what I discussed above. If cost is a concern, there are plenty of low cost facilities in your area. Most of them will also cover other low cost services such as vaccines, flea prevention, microchipping, deworming, FeLV/FIV testing.