Allomothering proves to be a beneficial reproducitve strategy. According to Coall & Hertwig (2010), cooperative breeding hypothesis is when not just the genetic parents care for the offspring. This wide population of caring adults ensure the health and safety of the offspring in a less-stressed environment.
Living past reproductive years is a anomaly to humans. In terms of evolution and genetics, why would this be? According to Coall & Hertwig (2010), surviving the stressors of childbirth itself and later relying on others for help in child rearing, ensure the success of providing for offspring. But being able to allomother postreproduction, also ensures reproductive fitness. Caring for grandchildren is still looking after one's genes.
Coall, D. & Hetwig, R. (2010). Grandparental investment: past, present, and future. Behaviorial and Brain Sciences, 33, 1-59.