Who needs to be attracted to the content the most?
A kindergarten student will probably not understand or be captivated by a technical drawing. Neither will a preteen boy enjoy a book with illustrations geared to preteen girls. A story must capture the audience that it's intended to reach.
Age differentiates the audience. So does geography. So does the surrounding social culture. So does the political atmosphere. So do major events in their lives. Parents who have lost children may look on things differently than parents who have not. By listing the intersecting sectors of society, a target audience emerges.
At times the many factors can confuse illustrators. Why can't we just make pictures? However, knowing how the target audience thinks and what they value can really aid the artist as he creates illustrations that have meaning for the audience. And if an audience finds meaning in an illustration, the story becomes all the more powerful in their eyes.