The advantages of coloring pages:
1) Self expression - coloring on a blank "canvas" (notepad), is a way for children and grownups alike to reveal themselves. You can tell a lot about the way an individual is feeling by the images that they draw, the colors that they use, and so on. It is important to provide children an opportunity to reveal themselves, and not all children reveal themselves through words and through writing, many use art.
2) Color acknowledgment
3) Therapy - for many people (myself included) coloring is healing. Regardless of whether it's scribbling, or coloring the "best photo ever", coloring can be a way to de-stress, after a busy morning of school work, unwind, and relax, after the tensions of a day at school or work.
4) Grip/Control - many children discover how to hold a pencil, pen, marker, or colored pencil, by very first knowing how to hold a crayon. The small muscles needed for penmanship later start to be developed while coloring.
6) Building motor skills
7) Focus - Taking note of a single job for a length of time is needed for coloring and for all sorts of things throughout one's life.
8) Limits - Another thing that children learn from coloring pages, with preprinted photos on them, is how to accept boundaries. While a toddler or young child might scribble all over a coloring sheet, without any regard for the boundaries (lines on the coloring page), as the child ages, they will start to respect those lines, and make an effort to color between them. While I encourage blank paper coloring free of charge expression as often as possible, for many preschoolers pre-printed coloring pages are their very first direct exposure to printed boundaries. This early direct exposure to boundaries in print, will be a big help when handwriting time occurs, and the child 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 mention for now, which is that coloring in the lines is a turning point, a sense of achievement, the primary step towards an effective academic profession for many children. For many children coloring in the lines is just as crucial as counting to 10, counting to 100, reciting the alphabet, discovering the multiplication facts, etc. It's a turning point that says "yes I can" do whatever I discover, and it offers children with pride, a sense of self worth, and helps them to feel accepted in a society that is often fast to judge, and slow to respond. This sense of achievement will carry them through life, and assist them not to give up so easily, when something brand-new comes along.