After writing with some regularity for the last two years, I see that writing helps both externally and internally. Externally, writing is communicating. You can persuade, explain, and entertain, all in service of gaining an audience. Internally, writing helps you develop ideas. By putting pen to paper, logical gaps are revealed. Connections are made between ideas.
Every piece of writing doesn't have to serve both externally and internally. One or the other is sufficient in order to write. It's useful to repurpose the internal writing for external reasons, that's the basis of the Build in Public philosophy.
Because most people only understand the communication side of writing, almost every writing course is focused on the craft of writing. Therefore, most people miss the internal benefits of writing. And those who don't have an interest in building an audience don't try writing at all. But everyone has an interest in developing ideas and can benefit from understanding the internal rewards of writing.
Try writing for the sole purpose of getting your ideas on paper. Don't worry if it sounds good. After you write your idea out, go over it and make sure it makes sense. Fill in the gaps.
Focusing on the craft of writing is superfluous if you're not trying to communicate with other people. There's no point in polishing words when the ideas are undeveloped. Your writing will naturally improve as you write and read more and especially as your ideas are clarified on paper.