If you're a blogger, you're already putting out great content on a regular basis. You have readers who care about what you write and maybe even respond with thoughtful comments. Why would you ever need a book when you have all that? Well, we'll give you a few reasons...

1.  It increases your credibility. Lavie Margolin found that publishing a book helped establish her as an expert, saying “I have been in over 40 media outlets since my book was published including The Wall Street Journal, CNN.com, Newsday and CBS MoneyWatch. When pitching a reporter, I leveraged my expertise as an author to get their attention.” As we shared in an earlier post on blogging for business, authors tend to have more Twitter followers as well. Being able to say you're a published author means something and if your book does well and you can add "best-selling" to that moniker, even more so. Furthermore, you might find as you dig further into your subject to write your book, you actually become more of an expert as well.
2. It's a source of revenue. Many bloggers have little to no way to monetize their writing. Google ads rarely produce enough (except for very top bloggers) and they put countless hours into their work only to find they still need a day job. It gives you something you can sell or, as Jane Sheeba at E-Junkie points out, you could always use it as a free promotion for your blog, consulting services or anything else you offer.
3. You already know how to do it. You already have a voice, a topic of expertise and great content. You're 90% of the way to having a book. Ali Luke on Copyblogger, lets his readers in on a secret, "Everything you know about blogging applies to ebooks too." The same rules apply for writing headlines (now chapter titles), the importance of formatting in a compelling way and, of course, of creating great content that people want to read and share.