By boosting your topical content, you increase the value and relevance of your site. The more new stuff you add, the more of an authority you make your site, and this helps to improve your overall search rankings. It can also help you obtain rankings for "long tailed search phrases".
Long tailed search phrases are those that are less commonly searched, and usually exceed 3 or 4 words in length - they are also very targeted to your product or service. Blogs are great for getting rankings on long tailed search phrases. If you see a long tailed search phrase you want to rank for, use the phrase as the post title (if it makes sense to do so) and write a good, content rich post about it and your ranking potential for this phrase will rise.
By writing blog posts with your target phrases in mind, you can often help improve your overall rankings by increasing the relevance of the site to that search phrase. Consider also linking to other relevant pages within your site from the post.
