Desexing our pets makes it easier for us to responsibly care for them, and increases the enjoyable activities we can do with our dogs. There are many health benefits to desexing, as well as positive effects on behaviour.
By desexing your pet there are also benefits to our society, in not contributing to the large numbers of unwanted puppies in animal shelters.
There are no benefits to your pet in allowing her to have a litter.

What is desexing?
Desexing of the female dog is commonly referred to as spaying, and involves surgical removal of the uterus and ovaries.
Desexing of the male dog is called castration, and involves surgical removal of both testicles.
Desexing is done as a day procedure, and is recommended when a male or female dog is approximately 5 months of age (prior to her first heat cycle for females).
The female dog comes into heat every 6-8 months or so. During a heat there is a bloody vaginal discharge and local male dogs are attracted. Male dogs may jump the fence to get to a female on heat, and females may have unwanted pregnancies despite their owner's best efforts.

What are the health benefits of desexing?
Females:
Mammary Cancer Prevention

A female dog spayed before her first heat will have almost no chance of developing mammary cancer.After the first heat, this incidence climbs to 7% and after the second heat the risk is 25% (one in four!). Desexing at 5 months of age can completely prevent what is frequently a very difficult and potentially fatal form of cancer.

Pyometra Prevention
Pyometra is a life-threatening infection of the uterus that generally occurs in middle-aged to older female dogs in the six weeks following heat. The uterus with pyometra swells dramatically and is filled with pus, bacteria, dying tissue, and toxins. Without treatment, a dog with pyometra will usually die. In most cases emergency surgery is required if a dog with pyometra is to be saved.
Pyometra is an extremely common disease of unspayed female dogs. Desexing prevents this pyometra.

Males:
Prostate problem prevention
Under the influence of testosterone the prostate gland will gradually enlarge over the course of the dog's life. In older dogs, it is likely to become uncomfortable, possibly being large enough to interfere with defecation. The prostate under the influence of testosterone is also predisposed to infection (prostatitis), which is very difficult to clear up without castration. Desexing causes the prostate to shrink, preventing both prostatitis as well as the uncomfortable benign hyperplasia (enlargement) that occurs with aging.
Other health benefits of desexing for males include the prevention of certain types of hernias and tumours of the testicles and anus. Excessive preputial discharge is also reduced by neutering.

Effects on Behaviour:
For males, the only behaviour changes that are observed after desexing relate to behaviours influenced by male hormones. Playfulness, friendliness, and socialization with humans are not changed. The behaviours that change are far less desirable. The interest in roaming is eliminated in 90% of neutered dogs. Aggressive behaviour against other male dogs is eliminated in 60% of castrated dogs. Urine marking is eliminated in 50% of castrated male dogs. Inappropriate mounting is eliminated in 70% of castrated dogs. While intact dogs may not show any signs of problem behaviour in the first couple of years of life, by 3 years of age, many will.
As the female dog's reproductive tract is dormant for most of the year (it only activates for the three-week period of heat), they act like a spayed dog most of the time. While they are on heat however there an increased risk of agression. Like entire males, intact female dogs may show erratic behaviour, and they may try to get out of the house or fenced yard while they are on heat.
When a female dog is on heat, both she and the intact males nearby will show behavioural changes, and many of the desexed dogs in the vicinity will, too. It can be difficult managing a female dog on heat.
Activity level and appetite generally do not change after a male dog is desexed, but a female dog's metabolism may slow slightly. Dogs of either sex should not become less interested in activity after desexing, or gain weight, provided an appropriate diet is fed. Some diet changes may be recommended for adult pets – your vet can advise you on this.

What if I want to breed with my dog?
If you choose to breed with your pet, it is important to be aware of the potential risks (and associated costs) that can occur if there are complications during the pregnancy or while giving birth.
Since dogs produce litters of multiple pups it's not necessary for a huge percentage of dogs to reproduce. Plenty of future dogs can come from the carefully selected dogs that live with people with time and talent to devote to responsible breeding. If this is what you want to do, find a reputable breeder to mentor you, and talk to your vet about tests that may need to be done to assess suitability for breeding.

When there is no good reason to keep a particular dog intact for breeding, desexing is a great way to keep your dog happy and healthy, and avoid unwanted litters.