You're assuming that a purchase is done the instant a company/person is found. What a blog does is build trust, familiarity and credibility in a particular field. So there's two scenarios : one is a reader becomes familiar with you and purchases a new offering (creating a channel and selling into it), the other is a reader finds your product, skips through some of your blog posts and decides to purchase based on the fact you appear to know what you are talking about. (creating credibility)

Of course this only works if your blog posts are related to your product offering.