How many times have you explained something to someone, only to be misunderstood because you couldn't find the right words? When this happens to me I always have the same thought; I should write about this.
To me, writing is the process of shaping my understanding of something. An idea, a perspective, a story — anything rooted in thought.
I believe when you transpose your thoughts to written words, you're simultaneously reflecting on the content itself, exposing yourself to flawed thinking or gaps in understanding. This provides the opportunity to clarify, or find the right delivery.
Say someone asked you to guide them to a place. Ideally, you'd have walked the path to that place beforehand, otherwise you might get lost. In mapping it out yourself first, you may discover a faster route, or another that's simply a dead end. Regardless, you've pushed the needle of your understanding forward, which will be reflected in your guidance.
Now consider again the example I opened with; explaining something to someone. The same idea applies, you're much less likely to get lost if you've already explored and clarified your thoughts on the matter.
I think this is why my brain yearns to write whenever I can't find the right words; I want to write, so that I can communicate, so that I can be understood. This is why I'm planning on writing more.