The following information is copied from here.

If the feeling you get from saving a dog from a kill-shelter is not enough, you’re really saving two lives when you adopt, not just one. When you adopt a dog you save that dog from potentially being euthanized, and at the same time, you free up kennel space for another dog to be brought in.

There are so many other reasons as to why you should adopt a dog.

1. When you adopt a dog, you have a better chance of finding the perfect match for your family. Shelter workers tend to have a pretty good grasp on the dog’s temperament, so they can help match you and your family with the perfect dog for your personality and your lifestyle. This is a great way to ensure that you don’t have to bring the dog back because he just wasn’t what you expected.
2. Shelter dogs are generally kept up with they’re vaccinations, spayed or neutered, and microchipped before or right after you adopt the dog. This helps cut vet bills tremendously. Yes, you have to pay an adoption fee, but that is generally a mere percent of what the shelter actually spent caring for the dog and vet bills, which means it’s just a mere percent that you have to pay for something that would have normally cost hundreds.
3. Most shelter dogs, depending on their size, are somewhat crate trained. Even the larger dogs in the dog runs are still considered somewhat crate trained, as they’ll generally potty on the outside portion of the run. This is a big help when you bring the dog home, as it reduces housetraining time.
4. Generally, shelter dogs bond rather quickly and deeply with their new owners. Usually, these dogs are uprooted from a happy home due to divorce, death, or some other lifestyle change, that they do go through a mourning process, so once they’ve found a loving home again, they want to please the new owners as much as possible to hopefully ensure that they won’t be homeless again. Even dogs who were picked up from the streets, never really knowing a good home, will bond quickly to hopefully ensure that they won’t be rummaging through the trash for their dinner.

On top of all that, when you adopt a dog from a rescue/shelter, you inevitably help reduce the number of puppies at the pet store. Remember those puppies are at minimum 90% puppy mill puppies.

It probably sounds like a stretch reducing the number of pet store puppies by adopting a dog, or puppy, but think about it. Most people adopt pure bred dogs because they’re under the impression that the only good dog is a pure bred dog or that pure bred dogs are the only ones worth having, so by adopting a good dog from a shelter, you’re decreasing the demand for pure bred pet store puppies, which will in turn eventually help decrease the supply (it’s the basic supply and demand concept).