The parents are in charge of 'bringing up' a child. This doesn't mean to say the grandparents aren't interested in the child's upbringing, but they recognize they have a different role to play. They don't have the authority they had as your parents; they have more of a complicity with their grandchildren which is an effective way of passing on their wisdom and supporting their grandchildren. Most grandmothers draw a clear line between their role and your role as a parent. They often see their role as that of another kindred soul who is always there.
Unless they have sole responsibility for their grandchildren (which is rare these days), grandparents aren't the ones who should be teaching and bringing up their grandchidlren. But sometimes parents and grandparents can have a difference of opinion over how to handle children, in which case their roles should be clearly set out. It's important that parents and grandparents complement each other, especially when chidlren become teenagers and can start to experience difficult relationships with their parents.
Learn with gran
A child's upbringing and education isn't just about homework and rules of thumb. They need more guidance and knowledge (more informal, but just as important) for their personalities to develop fully. Children grow through conversation with adults: parents, teachers and grandparents too! Grandparents are also really important for passing on their knowledge, wisdom and family history.