The life-blood of your web site (or your marketing efforts) is traffic targeted to your product. When you write blog posts, articles for publication in e-zines and e-reports you can also put them on your web site, and elsewhere on the web, like Facebook and LinkedIn. This way you are putting breadcrumbs across the internet, a pathway to your site/product.
By locating online publications that are already focusing on your target customers and putting your guest posts, or articles or free reports with them you are mining a well of prospects that are already leaning in your direction.
This way you are finding your market stream of prospects and stepping into a flow of prospects that are already there, instead of using a scatter-gun approach to finding those who already have an interest in what you have to offer.