Obedience classes teach YOU how to train your dog.
You train your dog so that he can be a happier, healthier, more well adjusted companion that you will be proud to take anywhere. A well trained dog comes when called - even when there is that tempting morsel just at dog level in the hand of your toddler. A well trained dog walks nicely on lead, even at the veterinarian's office. A well trained dog doesn't steal food, soil in the house, nip at people's heels or jump up on the guests. He can be found sleeping in a corner while the rest of the family enjoys dinner.
You train your dog because he deserves it. Because he doesn't want to spend his life in the garage or tied in the backyard. He wants to spend his life at your side. That's why you train your dog.