Do you have children? Raising children is hard. And it's a lot less hard when you have other adults to help. Historically, those would be grandparents and aunts/uncles. But aunts/uncles would probably have children of their own.
Nobody's saying that grandparents being alive are a prerequisite for grandchildren. But anybody with kids will tell you that grandparents are tremendously helpful in raising children.
Evolution is statistical in nature, not binary. Grandparents just have to be useful in raising grandchildren for them to be favored by natural selection. They don't need to be strictly necessary.
