The best writers never stop learning to write. And the best writers are not above taking in a new "Introduction to Composition" or Writing 101. There is always something to learn.
Across disciplines, some think that once they have taken an introductory course, or once they have "learned the ropes," the rest is specialization. While it is true that specialization is an important next step, it is never a waste of time to re-introduce your skills. From picking up an old grammar book, to a critical reading of the latest and greatest style guide, writers have to work to stay on top of their games.
This exercise takes a certain amount of modesty, and if you are unwilling to try it, you might wonder why. Are you really that good that you have nothing left to learn? Do you know everything in an introductory composition text book? Could you rewrite an introductory composition book? If you can't --- and few people can --- then there's nothing to be lost from giving it a go again.
Whenever we read anything, we change; good writing changes the reader. Doubtless you have changed a lot since your first composition class, and as a consequence, you will draw different lessons from taking it all in again.
Additionally, introductory courses have their own voice, and it is always a useful exercise to study voices. Perhaps after all these years, you find that an introductory text fails in certain ways --- ways that you would not have been able to identify in the past. It is valuable to be able to express this; it improves your understanding of your own skills. Or, it could be that as a less experienced writer, you were unable to grasp the important lessons of a text, and now you can. Then, obviously, you will be learning new techniques, styles, expressions, and so forth. Another valuable lesson.
You never know as much as you think. The world is ever-changing, and so is our concept of "knowing." "Knowing" is a moving target, since the world changes with us. Having a close look at where you have been clarifies your view of where you can go.
