A picture may be worth a thousand words, but only because humans are acutely visual animals. Large portions of our brains are devoted to making sense of the information we pick up with our eyes, processing changes in light and shadow, movement, and finding faces in everything we look at. (For more on that, here’s an interesting video from the MIT Museum -- Nancy Kanwisher talking about her research on human visual processing.) Visuals become particularly important in the sciences, where a well-drawn figure can be the difference between clarity and a confusing morass of conflicting data.
Amazing Rare Things: The Art of Natural History in the Age of Discovery and Howtoons: The Possibilities are Endless! are two books that are all about the figures.