Everyone’s saying so the parents don’t have to worry, but there’s also an evolutionary / natural selection aspect to it. Those that ensure their kids had kids are more likely to successfully ensure their genes are passed on than those that don’t. Also, grandparents can help alleviate the burden of raising a child and allow parents to make even more kids

https://www.smithsonianmag.com/science-nature/how-much-did-grandmothers-influence-human-evolution-180976665/