Drawing helps myself and my clients visualize the design.
I use sketches as another form of communication with my clients, craftspeople and builders. It's a great tool to show I what I mean when I say "rustic fireplace mantel" (for example), and it opens up a deeper level of dialogue for my client and myself about how they are going to use their environment. Sketches are such a great tool to hammer out the details, from dealing with unexpected support columns to adding windows and faux beams or a simple fireplace surround.
I am a visual person and I've found that most of my clients are as well.