There are approximately 3.5 million companion animals euthanized each year, most of which are healthy but homeless.
Every year in the US, according to a national survey by HSUS/Maddie's Fund, "you have roughly 17 million people who will acquire an animal but have not decided where that animal will come from."
While more and more are being adopted from shelters and rescue groups, there are still a great number of animals being purchased from irresponsible breeders, pet stores and puppy mills.
While not all breeders are irresponsible in and of themselves, there are too many purebreed dogs (and cats) in shelters and rescue groups that need homes. Please look to them first when choosing your next pet aka family member.
The ASPCA has some wonderful FAQs and guidelines about puppy mills and how pet stores hide behind USDA licensed breeders (who are often running puppy mills).
The standards that breeders (thus, puppy mills) are required to meet by law are extremely minimal. Under the federal Animal Welfare Act, it is completely legal to keep a dog in a cage only six inches longer than the dog in each direction, with a wire floor, stacked on top of another cage, for the dog's entire life. Conditions that most people would consider inhumane, or even cruel, are often totally legal. - ASPCA
An excellent article found on RadioFence.com about the harsh realities of puppy mills provides more insight into the miserable lives the breeding dogs (and cats) lead.
American Humane Society, Born Free U.S.A., Pet Finder via Statistic Brain
27 states and the District of Columbia have enacted laws which establish some form of humane care standards for animals kept at pet shops and sold in a retail environment
Source: Born Free; United With Animal Protection Institute