Parents are the front line of society. They bear the heavy burden of the work, but they are young, and they can take it. Their role is DO.
Grandparents are the back-ups of society. Their main function is to fill the gaps, to step in when emergencies occur. Their role is to HELP.
Great-grandparents are the great symbols of society. Whatever children, parents and grandparents may do, their lives are yet to some extent potential. For the most part, great-grandparents' tasks have been accomplished, but their role is still extremely important, because they represent everything the others hope to be. Their role is to BE.