Be known as the SME in your area

One of the goals of writing besides the satisfaction of having your own book is to be known as the expert on a subject. Having a book that you've written automatically makes you part of the discussion. This is important not only for book sales of your current book, but future books as well.
Writing a book about a compelling topic almost pleads for a follow up or sequel to the original.
Several authors have used the original book as the cornerstone of their marketing and sales. Being seen as an expert on a topic opens up speaking engagements and opportunities to market several items in your arsenal of products.