The more pets spayed or neutered, the fewer cats and dogs will have to be destroyed. No family wants to cope with an unwanted litter. Spaying prevents your pet from giving birth to unwanted kittens or puppies. Males neutered early in life are less aggressive toward other males and are not distracted by females in heat. A neutered male will be less tempted to leave your property and cross that dangerous street searching for a mate.
Neutered males are also less likely to mark territory with their urine.
Spaying your female pet eliminates the problem of stray males camping in your yard and decreases her desire to roam and breed. My pet will get fat and lazy. Neutering or spaying may diminish your pet's natural tendency to wander, but will not effect the overall activity level. When pets do gain weight after being altered, it is usually attributed to a combination of overfeeding and inactivity. Just remember to adjust the amount of feeding to your pet's activity level.