A blog can establish your identity as a writer in two ways; by raising awareness of your work, and by raising awareness of yourself. Let's start with the work itself. Whether you are still looking for representation or are already published it is difficult to get yourself noticed in an overcrowded marketplace. In fiction, the buying public is reluctant to try something new and in non-fiction the internet is such a vast source of information that people are buying fewer books for reference. By blogging about the process of writing a book, the problems and pitfalls, you can create a sense of anticipation for that book's arrival. By blogging on the same subject as your book (assuming it is non-fiction) you can establish yourself as an expert; contact websites on that subject and see if they will post a link to your blog (you can return the favor with a link to their site). Writers are often solitary people who may not enjoy the limelight. Because of this writers sometimes forget that they themselves are a marketable commodity. Everyone wants their work to speak for them and, in the end, nothing is more important than the quality of the work, but the work is coming from you and if you can make yourself part of the sales pitch then it gives your work a boost. A blog allows you to do this. How are you qualified to write what you write? What inspired this story? Do you sit down to write like it's a job or do you write when the mood takes you? You can even talk about your personal life (if you are comfortable doing so), though how much you include is a personal choice. Above all, in these times of financial austerity buying a book is becoming almost a luxury and people are unwilling to take a chance on an unknown author. Your blog lets readers get an idea of your writing style for free. If they like it they might be willing to spend some money. Likewise your blog can give an agent or publisher an idea of your writing style, so make sure the blog is properly marketed on Facebook, Twitter, etc. and that you have linked it to as many relevant sites as you can, as well as putting the site address on your resume.