This is a question that I am asked by my clients and prospective clients all of the time. I respond by giving them two primary reasons, both of which are helpful when it comes to SEO: a) it’s an easy way to provide fresh content, b) it improves your site’s SEO for long-tail keywords.
They usually understand the whole “fresh content” thing, but most of the time they are perplexed about the long-tail term thing. Long-tail keywords, in the most simplistic definition, is search someone enters into a search engine containing more than three or more words. Most people are in an advanced buying stage when searching using a long-tail keyword, therefore they have a very high conversion rate. An example would be “red and blue bicycles for sale in denver under $100.”
According to Wikipedia, long-tail keywords often have a higher conversion rate of up to 200% compare to simple or generic one- to two-word keywords.
It is often impractical to create an entire page of content devoted to certain long-tail keywords because most aren’t very competitive due to how infrequently they are searched for, and a simple blog post may be enough to push your site to the top of the search results. But, again, even though these terms may not be used frequently, when they are used there is a high probability the click you receive from the organic search result will end in a sale or conversion of some kind.