There are many reasons for a writer to blog.

Research – Blogs can be a great tool for researching your book.  This can work in a couple of different ways.  Your own blog can be used to solicit and collect research information.  Or, you can use an RSS reader to track blogs which are discussing topics pertinent to your book.

Findability – Blogs tend to do well in search engines because of the frequency of update, copious tagging through the blog software itself as well as the greater number of inbound and outbound links.  These and other factors will usually help ensure your blog posts move  up more rapidly in search results.

Writing discipline – Blogs can help build writing discipline.  Many writers struggle with getting into the practice of writing every day, or some regular schedule.  With blogs, you can see the readership start to build; generally, the more often you blogs, the faster the audience will build.  This can be a great stimulus to writing every day. 

Audience building- Blogs are wonderful audience building machines.  First, they are more findable (see item above).  Secondd, when others link to your blog or its posts, those links act like traffic annuities, continually bringing more visitors to your site.  You can amplify your blog’s audience building potential through RSS subscriptions, bookmarking sites and social networks like Facebook.

Measurability- Most blogging software now comes with a set of basic metrics built in – e.g. number of visitors and pageviews.  But through the use of tools like Google Analytics or other online measurement services, you can track an amazing amount of information about visitor behavior and engagement – e.g. where visitors are located, what sites referred them, how long they stay on your site, what they read or download, and so on.  This information can be used with an agent or a publisher to demonstrate who the audience is for your content and what content they like best.  Referencing this data can make your book pitch much more successful.  If you are self-publishing it provides valuable insights into your potential market.

Content selection – Examining the metrics you’ve collected for individual blog posts can tell you which content is the most popular with your readership.  It becomes a straightforward matter to weed out content that doesn’t draw an audience and include more content that does.

Marketing- Finally, blogs make a great marketing platform.  You can have your blog do double duty as a book marketing site – including things like: speaking events, appearances, reviews, testimonials and links to sites where the book may be purchased.  Many authors also now do blog tours.  These are similar to book tours, except that instead of going to cities and speaking , they visit blogs focusing on topics related to their book.  The author provides an interview or the blog owner agrees to review the author’s book.

In essence, blogs can help you build an audience for your work while you write and then market your title back to that same audience.