I think it can be instructive to look at other social mammals that range over large areas, like Orca and Elephants. They tend to lean heavily on elder females as the purveyors of tribal knowledge and inter-generational memory to remember where to go and how to get there as environmental conditions vary from year to year and decade to decade. It doesn't matter as much in humans since we have writing and maps now, but this kind of wisdom of those who remember similar lean times from 60 years ago could have been very important to the survival of human communities.
Happy accident at first, though there are some possibilities around grandparents increasing survival rates of younger children, so that would ultimately feed back into the gene pool.
Clearly living longer after viable reproductive age offers something. Having grandmothers around to nurture the second generation offspring is certainly a benefit.