Sometimes it comes quite handy, if you use families to select a group of objects which should be animated together but can not be grouped: In example, if you would like to to rotate the blades around their own axis, then you have to animate them seperately in object coordinates.

In this example the turbine blades are linked to the turbine hub (which is itself linked to the case). Their origin handle is moved to the turbine hub and they are all in one family, so they easily can be selected and animated separately without interfering with the normal animation.