Given the limitations of short-term memory, we externalize our thoughts in order to see them, share them, and scrutinize them. This takes the form of writing (description), drawing (depiction), and gesturing (demonstration).
Through one (or all three modes combined) we communicate or think 'out loud' so to speak. For this reason, it's important to realize the power of writing when writing might be the fastest and easiest way to capture an idea. In another instance, acting out a scenario to 'feel' the experience, makes the best sense. And of course drawing, whether quick basic maps or diagrams, or more elaborate artifacts, helps the designer to 'think' and that's what it's all about.