http://blog.wolfire.com/2009/02/why-game-designers-should-learn-to-draw/
If you’re a professional game designer, it’s essential to learn about the basics of every aspect of game development! Every discipline is important, but today I will focus on why it’s helpful to understand visual artwork.
Concept Art
Having a working knowledge of 2D concept art can give you much higher-bandwidth communication with artists. For example, Aubrey is an awesome artist, but he can’t read my mind; if he draws a character or object that is not compatible with the design I am thinking of, we need to be able to speak the same language in order to change the design and the character and bring them closer together. Sometimes I have ideas or suggestions that can be communicated more efficiently with simple sketches than with words. Here’s an example of a ‘greenline’ drawing I did when Ian Wells was working on the old rabbit model.
Also, drawing reference images is an effective way to make sure that you have looked at and considered every part of the image in detail, which can help come up with ideas for parts of the game design. I was having trouble thinking of interesting ways to make the rabbit characters look different, so I sketched some rabbit faces to get a better idea of how they work.