It keeps your dog healthy!
Walking you dog regularly ensures that your dog's body weight and condition stays tip-top. Burning those calories, in addition to a healthy diet, ensures that your dog's weight stays under control, and that those muscles get a good work-out. If you have a particularly energetic dog, such as your typical Kelpie or Cattle Dog etc., you could even combine you and your best friend's exercise routine with your own. I take my dog along for run's and often take them along to my local Parkrun (Take note that they only allow one dog per runner). Please be aware that unlike humans, dogs don't have sweat glands like we do, so take water along, and make sure they stay cool and hydrated.
I also found that it has profound benefits to your dog's joint health. I have a Tibetan Spaniel, that has this problem with his knee joint popping out, pattelar luxation, I think is the fancy term. As his fitness level has increased, we noticed the problem reduced significantly, and I can't recall this happening with him in the past two years at all.
It also keeps your dogs digestive tract in good order. You'll notice that your dog suffers less from constipation when he goes for regular walks. Dogs also like to empty their bladders on walks, so that should help keeping urinary tract infections at bay.
Your dog will be happier, and behave better!
Walking gives your dog the opportunity to interact with other dogs, and learn some of those body language cues, that they can only learn from interacting with other dogs. Our little dog for instance had the bad habit of snapping at bigger dogs when they tried to smell his butt, which made them in turn behave more aggressively towards him. He has now learned to relax a great deal when these encounters happen, and respond correctly to the social cues. I've seen some overexcited dogs jump up onto the bags of other dogs, which they interpret as an attempt at asserting dominance, which leads to aggression. It is therefore important to negotiate with other dog walkers before allowing your dogs to interact on walks. I am a great fan of Cesar Milan's and would recommend watching some of his YouTube videos to prepare your dog for socialization.
You'll also find that walking gives you a lot of opportunities to train your dog. I have been able to train my dogs to walk beside me, and to recall them when off leash without much dog training experience. Being in control of your dog makes you a much more relaxed walker, and makes dog walking a pleasant experience.
It also curbs boredom, which is, I believe one of the major causes of badly behaved dogs. Chewing through items in your backyard (I still remember how my dad's Rottweilers used to fish the pool cleaner out of the pool to his dismay), excessive barking, overly aggressive responses to visitor's or passersby are often due to dogs being bored! You'll find that your dog is a lot calmer and exhibit more self-control when he gets to go for regular walks.
Your own Mental Health!
If you have a less excitable/hyperactive pet to deal with at home, you yourself will have a more pleasant experience with your pets, and it will reduce the potential for conflicts within your household, and potentially with your neighbours. So don't delay, get your dog a good leash and hit the road!