Training classes actually are for the dog as well. I know how to train a dog

but there are always new ideas, different opinions, and different methods one can be open to.
Primarily it's for socialization and proofing the dog around a large group of other people and dogs and in a different enviornment. You can take them to the petstore and park all you want, but nothing beats the socialization and proofing that training in a large active group does.