The advantages of coloring pages:
1) Self expression - coloring on a blank "canvas" (notepad), is a method for kids and grownups alike to reveal themselves. You can inform a lot about the way a person is feeling by the images that they draw, the colors that they use, etc. It is very important to provide kids an opportunity to reveal themselves, and not all kids reveal themselves through words and through writing, lots of usage art.
2) Color recognition
3) Therapy - for many people (myself consisted of) coloring is healing. Regardless of whether it's doodling, or coloring the "best image ever", coloring can be a method to de-stress, after a busy morning of school work, wind down, and calm down, after the stresses of a day at school or work.
4) Grip/Control - lots of kids learn how to hold a pencil, pen, marker, or colored pencil, by first knowing how to hold a crayon. The little muscles needed for penmanship later start to be developed while coloring.
6) Structure motor skills
7) Focus - Paying attention to a single task for a length of time is necessary for coloring and for all sorts of things throughout one's life.
8) Boundaries - Another thing that kids gain from coloring pages, with preprinted photos on them, is how to accept boundaries. While a young child or young child might doodle all over a coloring sheet, with no respect for the boundaries (lines on the coloring page), as the kid grows older, they will start to respect those lines, and make an effort to color in between them. While I encourage blank paper coloring for free expression as often as possible, for lots of young children pre-printed coloring pages are their first direct exposure to printed boundaries. This early direct exposure to boundaries in print, will be a big aid when handwriting time comes around, and the kid needs to respect the boundaries of the preprinted handwriting lines on the paper.
9) Milestone - This is the last little "importance" of coloring that I will point out for now, and that is that coloring in the lines is a turning point, a sense of accomplishment, the first step towards an effective academic career for lots of kids. For lots of kids coloring in the lines is just as important as counting to 10, counting to 100, reciting the alphabet, discovering the reproduction truths, and so forth. It's a turning point that says "yes I can" do whatever I encounter, and it offers kids with pride, a sense of self worth, and assists them to feel accepted in a society that is often fast to judge, and slow to react. This sense of accomplishment will bring them through life, and help them not to give up so easily, when something new occurs.