Ideas can be abstract or invisible and that makes them harder to work with. Visuals help make them concrete and (obviously!) visible. Then they're much easier to work with and evaluate.
Try this easy exercise:
Take a black and white design you drew a while ago.* Imagine the entire image fully colored in at least 4 different colors. Try to see the entire image with as much detail and as vividly as possible. Take your time and wait to get a really clear picture in your head before moving on. Then, color the image in as you pictured it. Odds are, you'll probably change your plan a bit as you go along. When things become visible and tangible, it becomes much clearer what works and what doesn't.
*Check out the previous post on Visual Thinking Practice: Thick & Thin Lines and Concentric Shapes to see how to make a design like this one.