A female cat can produce up to 24 kittens a year (with an average of 12) and a male many more.
The most obvious reason is to prevent unwanted animals coming into the world. There are enough animals needing loving homes already without adding to that number and many end up homeless and diseased, while still more are cruelly disposed of or unnecessarily euthanised due to overpopulation.
If an animal has been desexed they are less likely to wander in search of a mate so will be more settled and safe. They are also less likely to catch diseases that are spread from mating or fighting as desexed males do not need to fight over females.
Temperament and behaviour are generally better when an animal has been neutered or spayed. They tend to be more affectionate, settled and are less likely to mark their territory.
There is evidence to show there are a number of health benefits for animals when desexed. Among these are a greater resistance to infections and a decreased likelihood of some cancers.
Early desexing leads to quicker recovery for your pet. The recommended age for cats is between 4 and 6 months.
Exponential effect of not desexing