Why use coloring books? Well because they give me a quick and very clean drawing to work from and the art is usually pretty simple. It allows a lot of room to play and explore new techniques, textures, lighting and also makes me draw stuff I might not normally thought of myself. I would probably never draw fairies or flowers but here you go...
Nib over Tinkerbell
Winser Newton Series 7 over Toy Story
Faber-Castell PITT artist pen over Tangled