Coloring books are where most kids start to develop their artistic dexterity. They are available relatively cheap, they are often related to cartoon characters or objects familiar to very young children. They provide guidelines for children who are learning to control their hand movements by "coloring inside the lines".
The lowly printable coloring page is really a chance to show kids a lot of things about the world and best of all you can find coloring pages on just about every theme and idea and they are often free to print out. Printable coloring pages even work well in a pinch. If you have a bunch of kids over and only one unused coloring book left, it's easy enough to go online and print out as many copies of coloring pages as you need.
All the reasons listed above prove that coloring pages is a great educational tool for children.