Daily walking provide syour dog with positive attention from you and a chance to be with you doing something he loves. Dogs do not self-entertain. If you put your dog outside in your fenced-in yard, he will not exercise himself (unless there is another dog to play with). More likely, he will resort to boredom barking, trying to escape the yard or do some creative landscaping instead! As his guardian, it is your job to provide your dog with enough stimulation. If he spends 10 hours a day alone in the yard or in a crate while you work, do not expect him to be a well-mannered member of the family when you come home in the evening.
Dogs do best with a structured life and the daily walk helps to satisfy that need. Taking your dog for a walk is an important ritual in keeping your dog mentally stable. A dog, as an animal, is a walker/traveler by instinct. Packs of dogs get up in the morning and walk. Simply having a large back yard or taking your dog to the dog park is not going to satisfy this instinct in your dog.