Why did you write your book?

Financial gain. Profit from sales of your book is certainly a valid motivation, especially so if you intend to write as a career. Even if you are writing as a hobby, profitability can allow you to spend the time necessary to work on your writing and on promoting your book.
Establish and support credibility. Personal and professional esteem is enhanced by publishing a book that makes a credible contribution to your field of expertise. From a breakthrough concept to a compendium of existing knowledge, or an exploration of a specialty area within your field, a book tends to set the author apart as an expert.
Support and enhance other activities. A book can serve as the foundation for consulting, lectures, and other means of enhancing your current business. Book sales can be an integral part of providing these services or an additional source of profits. A book can also gain publicity, open avenues to new revenue, serve as a launching pad for appearances as an expert, and create new customers.
Expand beyond your field. Another way to enhance the perception of expertise is the book that demonstrates an in-depth knowledge of another, unrelated field. Consider, for instance, how political columnist George Will’s perception as a multi-dimensional person is positively affected by his books, commentaries, and documentary appearances as an expert on the sport of baseball. This shows the author as multi-dimensional, a well-rounded and wide-ranging intellect, and not limited to the subjects in which he or she is expected to be conversant.
Promote causes or views. Whether you are involved with political, social, or other activities, your point of view, ideas, and concepts about such things can often be promoted and spread more widely by a book on the subject, and by you as an author of such a book, than by other more conservative means. Consider that anyone can write a letter to the editor, or speak before a small gathering, but an author can present his or her views authoritatively to an almost unlimited audience.
Communicate special perspectives. If your ideas are not widely known, or you perhaps have an unpopular interpretation of ideas or events, you are not likely to be heard—those with whom you disagree or who feel challenged by what you have to say will do whatever they can to dismiss or ignore you. In book form, your ideas exist apart from your ability to personally communicate them, and can be much more widely distributed to those who may be interested in hearing what you have to say.
Focus on specialized interests. Even the most arcane subjects have their adherents, and through publication, they can be reached in considerable numbers regardless of where they may be located. Through the internet, any individual that reads English and has an interest related your own is a potential buyer of your book, in any country in the world and no matter how specialized your focus may be.
Make a literary contribution. Many of the books everyone knows today were originally rejected by scores of mainstream commercial publishers before finally being picked up by one that, for some reason, was willing to back the author’s work. Jonathan Livingston Seagull, for example, was finally published only because Richard Bach happened to have made a contact with an editor through a couple of his other books on aviation. How many other works, perhaps not destined to be bestsellers, but books of merit, don’t even exist today except as a rejected manuscript in a box or file cabinet?
Create a personal expression. Some books are meant to be published exactly as the author has envisioned them. They are not meant for the mass market, but for select distribution as a personal expression of an idea, one that must be produced according to the author’s ideas of structure, typography, page layout, and illustration. Self-publishing gives you virtually complete control over your final product.
Preserve personal knowledge. You may want to share special knowledge and insights with others, as in a family history, for instance, or a book on some aspect of regional lore. Your story may be something that you want to pass on to others long after you are no longer here to tell it, available in your own words for whoever may find it one day. Consider what your own children or grandchildren really know about you and your experiences. Your personal insights are unique, yet what you have learned over a lifetime may be only a footnote in the future, if it is remembered at all.
Personal enjoyment, creativity, self-expression. Perhaps the best reason of all to self-publish your writing is—just because you want to. Photographers and artists put their work on display as much for their own enjoyment as for that of others, or for the possibility of selling a piece of their work. Musicians create CDs of their music. The writer who creates for the enjoyment of it is no less justified in wanting to see his or her work in print than is the visual or performing artist.