As I understand this if you join the DVG club, you will be required to join DVG. You need to be a member to trial your dog. Some clubs include the DVG dues in their club dues; others handle it seperately. But a DVG affiliated club requires you to be a DVG member.
DVG works as an organization overseeing trials, certifying helpers and judges and tracking your accomplishments. (and collecting money....) You also get a quarterly magazine.