When Can Grandparents be just Grandparents?

It seems that is has become common placed for kids to have kids and Grandparents to raise them.  I've spoke with some Grandparents recently and they refer to themselves as parents.  Being called mom and dad by their grandchildren.  In some cases, this is understandable when there has been a death of a parent or other some other circumstance beyond one's control.  

But when one voluntarily decides not to care for their child, is it because they are too young, do not know how or is it for some other reason?

What are your thoughts on this subject?  We would love to hear from you.