Once the colors are put in their "wheel" students always comment....How pretty or cool  these are!". Why? Because visually they are perfect....just like the rainbow! 
What is the purpose of constructing a color wheel? Simply put, to set in your mind the structure of color and how  each is made. The three primary hues -- yellow, red, and blue -- are the basic building blocks of color. All other colors are derived from these three. The three secondary hues -- orange, violet, an green -- made of primary colors. And then  the six tertiary hues -- yellow-orange, red-orange, red-violet, blue-violet, blue-green, and yellow-green. The color wheel has a total of twelve hues, arranged like the numbers on the face of a clock.
 More on color theory: http://swegenas.aurora.esu9.org/midart/color_theory.htm