The benefits of coloring pages:
1) Self expression - coloring on a blank "canvas" (notepad), is a way for children and adults alike to reveal themselves. You can tell 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 offer children a chance to reveal themselves, and not all children reveal themselves through words and through writing, lots of use art.
2) Color acknowledgment
3) Treatment - for lots of people (myself included) coloring is restorative. Regardless of whether it's doodling, or coloring the "best photo ever", coloring can be a way to de-stress, after a hectic morning of school work, unwind, and cool down, after the stresses of a day at school or work.
4) Grip/Control - lots of children learn how to hold a pencil, pen, marker, or colored pencil, by first knowing how to hold a crayon. The small muscles required for penmanship later on start to be developed while coloring.
6) Structure motor skills
7) Focus - Taking note of a single task for a length of time is required for coloring and for all sorts of things throughout one's life.
8) Borders - Another thing that children gain from coloring pages, with preprinted pictures on them, is how to accept boundaries. While a toddler or preschooler may doodle all over a coloring sheet, with no respect for the boundaries (lines on the coloring page), as the child gets older, they will start to appreciate those lines, and make an effort to color in between them. While I encourage blank paper coloring totally free expression as often as possible, for lots of young children pre-printed coloring pages are their first exposure to printed boundaries. This early exposure to boundaries in print, will be a big aid when handwriting time comes around, and the child has to appreciate the boundaries of the preprinted handwriting lines on the paper.
9) Milestone - This is the last little "significance" of coloring that I will mention in the meantime, and that is that coloring in the lines is a turning point, a sense of accomplishment, the primary step towards a successful scholastic career for lots of children. For lots of children coloring in the lines is just as crucial as counting to 10, counting to 100, reciting the alphabet, finding out the multiplication realities, etc. It's a turning point that says "yes I can" do whatever I discover, and it provides children with pride, a sense of self worth, and helps them to feel accepted in a society that is often quick to judge, and slow to respond. This sense of accomplishment will bring them through life, and help them not to quit so easily, when something new comes along.