Writing helps you think about your ideas. It also publicizes your ideas. Writing is the first step to making ideas into reality. It helps form the idea while simultaneously building support.
One of the reasons it took me so long to leave my job was that I didn't know what I would do next. I also felt that I didn't have any skills that were useful. Writing (and doing) opens possibilities. The response from other people lets you know what they find useful. I started a newsletter and from sheer stubborness wrote 80 issues. Now people want to know how I did that. Exposing your ideas lets you know which ones are useful.
The biggest mistake people make is assuming that everything has to work out. Every thing you write needs to be right and persuasive. What I've found out is that ideas are dynamic. Nothing survives a first draft. After you write something, it could be ignored, evolved, more nuanced. But not exposing an idea leaves it in a primordial stage. These early stage ideas are not acted on. Action is needed to create movement. I didn't leave my job because I didn't have an actionable idea but I didn't want to do the work in getting an idea actionable because I was in my job.
People don't want to leave their job because they want to replace their job with something equally as certain. Even things that are certain don't seem certain when they start out. The most important precondition is an exploratory/experimental mindset.
Writing is a low cost way to develop an experimental mindset.