Dogs are social creatures, just like people, which is why it's important to allow your dog to play with other dogs starting at an early age. Just like children at school, this is how dogs learn how to interact with others of their kind. This is an easy part of a dog's development to overlook, since many people don't consider how important it is for dogs to get along with other dogs, so long as they get long with their human owners. However, interacting with other canines could help you raise a more well-behaved dog.
As many people already know, the rough and tumble play between puppies is how they figure out which dog is the "alpha dog." Although it is all in good fun, it also helps dogs figure out how they should behave in an environment outside the home. For example, they will learn what kind of behavior is accepted by other dogs and what is considered too aggressive. They will also learn to read the body language of other canines, and understand which dogs are behaving in a friendly manner and which ones are potential threats, and are better left alone. In a way, having social skills protects your dog, as he or she will learn how to "communicate" effectively with other dogs. It also helps your dog become more confident, more balanced, and less likely to react anxiously or in a threatening manner if they meet a other dogs in the future.
That said, it isn't enough to only socialize your dog when he or she is a puppy. Again, just like people, they need to see their friends and meet new ones even when they've grown into adults. Even if dogs were socialized as pups, they may still become unruly and aggressive if socialization is discontinued after they're grown.
Of course, some owners may argue, "But I have a small dog, I just won't let them out of the house. Ever. Then he won't know what he's missing." I actually knew someone like that, and to do that to your canine is quite cruel. Although we all consider our pets to be our companions, it's important to let them see the world and live their own lives as well! Allowing them the freedom to be among other dogs is not only fun and emotionally beneficial to them, but also challenges them and drains their energy when play is involved, therefore helping to keep them healthy, and takes a little pressure off of you - for that day at least - to make sure your dog is getting enough exercise.