Here is why ...
When you publish a good newsworthy release targeted to a specific group or industry that release will get picked up and syndicated by any number of news organizations from print to radio to internet news sites and industry specific websites.
The bonus factor from press releases is that each time your story is published on a website it creates a link back to your website thus dramatically increasing your link popularity a key to better search engine ranking.
The simple formula -
press releases = traditional and internet exposure + increased search engine ranking.
Incidentally... are you looking for a place to hone your press release skills and to distribute your release to over 100000 news outlets? Try
http://www.addpr.com