Most people blog because they want to inform the world of what they had for breakfast and their plans for the day. Those types of blogs aren't very popular though :P
Many serious bloggers using blogging as opposed to setting up a static website, as it tends to encourage repeat visitors as it is clear that the site is dynamic and likely to be updated soon if it is a blog. Since blogs are in chronological order, the new content is also easily accessible and doesn't
require sorting through large sections of old content just to find the new.
For sales purposes, some people discuss subjects about or related to the product they are selling. This creates a larger pool of repeat visitors to your site and hence encourages more people to purchase whatever product or service you happen to be selling.
Hopefully that answers your question

Let me know if it doesn't.