The world around us is in color. Our photographs reflect the world around us and the colors of the world (and the light) greatly affect the outcome of our photographs.
The color wheel tells us what colors blend well with other colors and which colors clash. Using the color wheel allows us to better control how the colors in our compositions affect the feel of our photographs.
Light has color to it. Which color is determined in part by temperature. By knowing the temperature of different light colors we can better predict what our light sources will do to our photographs.