Google attaches a lot of importance to blog posts. If a blog post contains a link to a company then this is very good in the eyes of Google.
Some internet marketing companies set up blogs purely from a commercial standpoint ie they charge to write a blog post that links back to a client's website.
The Office of Fair Trading has recently clamped down on a London based company that utilises this tactic.
In a press release the OFT stated that blog owners "must clearly identify when promotions and editorial comment have been paid for, so that consumers are not misled"
The full press release can be read here:
http://www.oft.gov.uk/news-and-updates/press/2010/134-10