Walking provides several benefits:
-Socialization: your dog becomes familiar with and non-reactive to a huge number of stimuli. People of all shapes, sizes, ages, races and levels of disability; normal environmental stimuli-bicycles, rolling trashcans, buses, fire engines, sirens, road equipment, people on ladders, etc.; dogs of both the polite and impolite sorts.
-Exercise. Admittedly, you can provide exercise without walks though.
-Entertainment. Dogs find walks exceedingly entertaining as they explore the world through smell. Even changing the side of the street on which you walk your dog opens up a whole new world for him as he acquires information so much more by smell than we humans do!
-Bonding and training. The dog enjoys walks, and if he enjoys them with you, you are bonding. And walks provide endless training opportunities! All sorts of things that you didn't anticipate (along with those you reasonable would) occur on walks. You get to practice giving your dog direction with high distraction and increase his trust in you and his obedience.
The "worst that can happen" if you don't walk the dog is that you have a fearful or aggressive unsocialized dog who only listens to you when there are no distractions present.
Yes, I know people who never walk their dogs and they are badly trained (or more accurately, half-trained) and usually fearful or aggressive.