I find writing to fulfill a few functions in my own life: catharsis, structure, and deceleration. But, they all they stem from the same issue I have in my life, and I believe many others also feel:
Information Overload. In a hyper connected world, we are constantly bombarded by input from the world. A Slack message, an e-mail, a text, an app notification, even scrolling through Twitter consuming nano-thoughts that naturally have a mental response in your brain. Our brains simply were never evolved for information intake at such a rate. Which is why it is so difficult to sit down and actually produce deep and meaningful work before being naturally interrupted by our lives again.
I haven't found any solutions to my monkey brain for being so reactive to all of this world's input. But I have found a way to make sense of it. And that is writing.
It's cathartic because I often need to release my own thoughts from my brain rather than have the world give me more things to respond to. This provides a tangible way for me to do so.
It provides me structure because I can actually SEE my thoughts. I believe this is an underrated aspect I overlooked for a long time. We have a swirling concoction of thoughts that all sort of interweave in and out of one another and we can't seem to separate any of them. But if you actually sit down and write you are literally able to see those thoughts as written sentences and distinguish one thought from another.
Lastly, deceleration. I've always been the person whose brain moved faster than this hands, therefore putting pen to paper or typing on a keyboard actually forces me brain to become rate-limited by my ability to produce words on a medium. I think slower, but deeper.
Journaling is my #1 recommended habit for any of my friends who deal with Information Overload. And please don't turn away from writing because you believe it has to be a public endeavor. Pick up a journal, find a pen, and just write what comes to your mind. Over time you'll be able to understand your thoughts in a clearer way.
But by creating a mechanism for yourself to think clearly, you create an ability and a skill to communicate those ideas into the world in a way others may understand.