Think of your blog as a better, more immediate, version of the monthly email newsletter. First of all there are no time constraints. If you have news, blog it now, not next month. And you don't have to wait until you have a pile of news to publish. Have one item? Post it.
From a marketing perspective, it makes little sense to wait a month to communicate with customers when you can do so now. Because blog posts are short, you can take one article, slice and dice it, and post bits of it every day.
Blogs boost your Google rankings. This is because each blog post is new content, so search engines rate the post higher than a static email newsletter page. Email newsletters sometimes have a difficult time getting through spam blockers and antivirus software but blog announcements don't.
Blogs can spread via syndication and social media. Your blog can provide an RSS feed that can be used by blog aggregators, news sites and other feed-reading mechanisms. They can also leverage social networks such as Facebook and YouTube, to increase your footprint even further. On the other hand, email newsletters are limited in reach to the number of customers on your mailing list.
Blogs can strengthen your online presence within Vertical Markets.
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Finally, a blog can double as an email newsletter! You can bundle up your blog posts for the last week or month and send them out as an email newsletter.