Often a writer will find themselves bursting with ideas and words that they must put down on paper for fear that those words, and the information that is contained within those sentences, will end up being lost forever.
Sometimes a writer is someone who is unable to convey information through speech in an accurate manner so would rather take their time and plan each word, to ensure accuracy, in the form of writing. Some writers will also lead the way and write about things that are designed to create a response or some other form of feedback with the added bonus of gaining recognition for the information that they have given.
All these things are also true of a martial art teacher who will find it in their nature to put across, not only information, but also concepts and ideas in the form of physical instruction to the student in just the same way as a writer would do. It is no surprise therefore that many teachers of the martial arts will also turn to instructional and creative writing in time.