I would argue that oil paintings or statues are equally synthetic. But my point is that these pictures are best used in education (hence the title).
Key objectives of education are to educate (of course - with original materials!), but also to inspire and motivate. The latter two can be best achiieved when audience can relate to the people in the stories - and these pictures can achieve that much better than a marble statue or an oil painting from a few hundred years ago.