Adopting from dog shelters is a kindness on many levels. First, you may very well be saving a life, since in many countries around the world — including the United States — shelters are forced to deal with overpopulation by euthanizing dogs and cats.
Adopting is also cheaper than buying. You will have to pay an adoption fee, sure, but that price also includes the cost of vaccination and the cost of neutering/spraying the dog. And those fees help keep the shelter open, so that they may take care of many other animals. Please consider doing regular donations to your local shelter if you have some income to spare.
Finally, you have all the reasons to get a dog in general. Dogs are loyal and loving friends that can make for a wonderful company. They have been linked to better mental and physical health among owners, and they can help improve home security just by being there. If you are sold and want to learn more about dog adoption, check out this World of Puppies article.