Explanation I've heard in popscience books:

Healthy grandparents that are around to support their children and take care of grandchildren increase the fitness of the entire lineage by helping their children have more children and those grandchildren to be healthier/safer.

You can make it broader and simplify:

If you are interacting with a carrier of your genes at all while they still might reproduce, you are having an impact on their fitness and thus evolutionary pressure exists.