The temperament a dog has as an adult is often different than what you would have seen in the same dog as a puppy. You also get to see the physical characteristics of a full-grown dog. You know exactly what size the dog is going to be. That can make it easier to make a good choice. Plus, aside from getting a great companion, you just plain feel good about adopting a grown dog whose fate is otherwise uncertain at best.
Adult dogs require less work than puppies do.
As cute as puppies are, they are a lot of work. Aside from having to be housetrained, puppies teethe, chew, and need much more exercise and attention than adult dogs. And the work doesn't last for just a few weeks. Many breeds have the characteristics of puppies until they are well over two years old. Retired racers are great house mates.