Lots of shelters are kill shelters, when adopting from a kill shelter people give the animals a chance to live a fuller life. Also adoption and rescue is not as pricey as buying from breeders. Another major reason isif someone wants to skip the hassle of puppy or kitten training, there are a varity of older dogs to choose from.
INTERESTING FACT: Each year 8 to 12 million dogs, cats, puppies and kittens are euthanized because there are simply not enough homes for them.