Each year, it's estimated that more than one million adoptable dogs and cats are euthanized in the United States, simply because too many pets come into shelters and too few people consider adoption when looking for a pet. The number of euthanized animals could be reduced dramatically if more people adopted pets instead of buying them. When you adopt, you save a loving animal by making them part of your family and open up shelter space for another animal who might desperately need it.
Animal shelters and rescue groups are brimming with happy, healthy pets just waiting for someone to take them home. Most shelter pets wound up there because of a human problem like a move or a divorce, not because the animals did anything wrong. Many are already house-trained and used to living with families
Usually when you adopt a pet, the cost of spay/neuter, first vaccinations (and sometimes even microchipping!) is included in the adoption price, which can save you some of the up front costs of adding a new member to your family. Depending on the animal, you may also save on housebreaking and training expenses.
Source: The Humane Society of the United States