Writing is not the product of thinking
For many people, writing seems like a daunting task.
This is unfortunate, since getting through life often involves writing: writing your thesis for an undergraduate degree, writing a report at a job, writing an email to a prospective business partner.
Writing requires clarifying your thoughts into concrete enough terms as to communicate with other people. In doing so, you are also clarifying your own thinking to yourself.
Clear writing requires clear thinking. Putting thoughts and arguments into written form leaves no place for wishful thinking and unfounded beliefs to hide.
A third element, in addition to thinking and writing, is learning.
During our education, we are often encouraged to write, not just to think about it. This is not by accident: thinking, writing and learning are all intertwined.