As a novel writer, I believe that it is a good idea to draw pictures. Any author would be well served to have a defined mental picture of your characters that you can refer to while writing. It is also a good idea because you can do as I do and occasionally release them as promotional material. If you are a science fiction and fantasy writer and have created an entirely new world for you characters to inhabit then it is a good idea to draw pictures of those created worlds. That way you have a visual image that you can refer to when discussing that world in your novel.
Even if you are not good at drawing, it is a good idea to have some form of picture of your characters and your setting. Whether you take inspiration for you character from a television show or movie, it is a good idea to keep a picture of that person readily available. When the referring to that character or location while on the third or fourth book in the series and the details begin to get a little fuzzy, you always have a solid picture to rely on. Because, let’s face it, when you write a lot then not every detail will be remembered with complete accuracy over the course of a long series. The last thing you need is for your main character to start the series with blue eyes and then by the fifth book have green eyes, for example. That is a detail that the reader will pick up on and violently jar them out of your story and possibly cause them to fall out of love with your book and with you as a writer.