To write is to think
Besides writing just for the sake of it or improving ones writing skills there are three different reasons to write: to convey knowledge, to express oneself or to think.
First two are obvious. They are meant for others to read. But the last one doesn't require an audience. Writing can be a powerful tool to help you think.
In the end all other forms of writing must start with the writing to think. To out it the other way around it is quite impossible to write without thinking. It's just a matter of expression and refinement if the end result will be of any use for others.