There are many reasons to write a book.
Some are enhanced credibility, to leave a legacy, to help other people, to make money, and a few more.
The book will bring you more credibility than anything else you can do. You can invest your time in writing a screenplay, publishing a line of greeting cards, or teaching a Workshop. All are viable contributions to society but none will give you the credibility of being a published author.
People think if you wrote a book you know something. And you probably do because in order to write a book you not only use your accumulated knowledge, you read everything else that has ever been published on your subject. You study other books, magazine articles, websites, blogs, interview experts, and so on. You take that information, distill it down, focus it to a particular type of individual reader, and give them the answers they are paying you for.
In effect, you’re giving yourself an advanced degree in your subject. You are giving yourself a PhD. After all, you are doing the research and you are writing the paper.
Dan Poynter/Para-Publishing