Yeah like snakeair said, it's a way of offering free advice to your customers/potential customers and gain their trust by establishing yourself as an expert.
If you add quality content and do your keyword research you can get your blog pages ranking for keywords and use them to target keywords that would be difficult to target with your home page.
All that content in your blog will help your sites PR
as it will be filling it with decent content.
Decent content will attract links from other blogs, share links in your posts to other bloggers and chances are they will link back, if you can get a blogger with 1000s of subscribers to link back to you then that's 1000s of potential customers.
You can also use your blog for list building.