An average female dog will have 2 seasons a year. Each will last approx. 3 weeks. If you wish to avoid puppies they need to be kept away from male company during this time. This means that more than 10% of their life is spent 'locked up' to avoid unwanted puppies. This can be avoided by 'speying' (carrying out a hysterectomy). It has also been shown that the incidence of mamary tumours (breast cancer) is much lower in bitches neutered before the first season. Incidence increases slightly after the first season, then the risk grows rapidly as the bithch goes through more hormone cycles.
Speying also removes the risk of a condition known as Pyometra (the uterus fills with pus and the blood supply to the utorus transports poisons around the body) which can be fatal.
For males castration reduces the risk of prostatic diseases and removes the risk of testicular cancer. Castrated males are less aggressive, less inclined to wander, less inclined to mark their territory and less inclided to unwanted sexual behaviours.