I have always wondered over the necessary of dog training classes.

I have come to the conclusion that it depends entirely on the dog and your routine with him:

If you have a puppy that does not get much socialization during walks then you should probably attend a class alongside your use of a dog training manual. On the other hand if training is going well and your dog is getting to learn through play with other dogs at the park or whilst walking then they are probably not necessary.

If you have an older dog that is causing you concern over his behavior, aggression or otherwise then you should seek the advice of a qualified trainer after you have tried to resolve the issue yourself through training.