Google sees the site better out of the box (98% of all searches are done on Google), you have more control of your site and it simply offers interactive elements that a regular html site does not always have. Not that they can’t be added in some way, but with WordPress they are integrated directly within it’s blog infrastructure.
Basic SEO plugins can be added along with social networking (standard in WordPress Themes now) to make it easy to get connected.

The site modification/addition interface is easy to access online and does not require any software to make changes.

These sites are a great resource:

http://herenextyear.com/35-reasons-why-wordpress-website-is-better-than-traditional-web-site-for-your-business.php

http://blairwilliams.com/2009/04/17/5-reasons-why-wordpress-websites-are-better/