With online printable coloring pages you never have to keep volumes of coloring books around. Most children change what they would like to color often. They bounce around from one theme or one set of characters to the next. Therefore they may abandon that coloring book with princesses or cars days after you buy it. Printing out the pages though allows them to always get what they want.
You can find many clip arts with slightest modifications yet with different subject matters either, thus there is a continuing process of preparing, producing and using of these school clip arts by a lot of people. With everything else changing so fast, I believe it is valuable to retain some things essentially as they have "always" been. If your little one is fascinated by Disney characters such as Donald Duck, Mickey Mouse or Tinker Bell, you can selectively choose to print coloring-pages on this particular theme. Cartoon characters tend to constantly fall in and out of fashion. If the current trend is that of Ben 10 or the Incredible, you can easily generate hundreds of printable coloring sheet based only on these cartoon characters. The minuscule effort it takes to compile an on demand coloring book is worth the smile on your child's lips.