The act of writing can be a good rehearsal for life itself. It can be a preliminary practice, a preparation for the conversation that we want to have, or don't want to. It can be a record of our values and what is important to us. Or it can serve as proof of our ever-changing nature.
At its best moments, we have thoughts in writing that we do not have otherwise. We enter a state of flow where thoughts just come, and those thoughts may be things that we have not considered or even things that make us uncomfortable. Writing can inspire our creativity and our courage.
And when we are done, we have something that we can look at, or reflect upon. We may decide to reject it before we show it to anyone else. Or we might use it to inspire us to do something that we never thought was possible.
I find writing every day is helpful for other reasons, too. Looking back, I can see what was important to me at a given point in time. And I can see how that changes.
In a way, then, writing is a record of our impermanence. When we think our view of the world is a constant, we can look at our writing and see that we are wrong. That given enough time, everything changes. Even, perhaps especially, our very view of self.