Not quite true as you put it, but the general principle applies. It's about voltage stacking and the same current being drawn from multiple sources.

Think of it as a slide going from the top of a building to the road below. You have a number of people that want to slide down (that would be the load). If you have one huge slide, the people will be hurting each other as they descend. Therefore, you add more paralel slides (batteries) so they can redistribute themselves and be less cramped (less overloaded system). By adding more batteries in series, you make the slope more steep so the people get down faster (more force to "push" them).