To the best of my knowledge, historically, whether called witnesses, sponsors, or godparents, their main duty was to ensure the moral and religious education of the child. In some times/cultures, their duties also included physical and financial responsibility in the event of the parents death before the child's majority. Quite often, even if the parents survived, a male child would be assisted in obtaining education, a job or apprenticeship. A female child would be helped to obtain a better marriage. Thus the practice of trying to get witnesses from a higher social/economic standing than the parents, especially for younger sons. -- Jo-Ann Croft