Illustrative material is valued by editors and publishers of reference materials and textbooks because it helps to make a page seem less intimidating to the reader. If a reader sees nothing but a large, uninterrupted block of text on the page, he will be more likely to think of it as boring and overly hard to read. By filling part of the page with visuals, and thus breaking up the text itself into smaller sections, the reader feels that he will be able to take each section of information in one at a time.
Especially at a young age, many learners have a more visual and concrete than abstract way of approaching the world. As such, graphs, charts, or pictures with bright-colors are more likely to draw their attention and keep it there for an extended period of time. For example, a picture of the human cell with the organelles given different colors will hold an 8th grader's gaze quite a bit longer than a text describing the same thing.
Many examples of illustrative material, such as graphs and charts, are specifically meant to help the reader remember certain information by presenting it in a more compact form. Furthermore, many charts and diagrams are based on the concept of "chunking," whereby information is broken down into smaller portions for easier memorization.
Pictures and illustrations of concepts in books provide concrete and literal examples of the material that will aid the reader in translating the things he has learned in the text to real world activities. For example, a text may be able to describe every fact a person could ever want to know about a particular species of South American monkey. But for a traveler hoping to spot one on vacation, all this information will be of little use if he has never seen a concrete image of the monkey in question.