1 - Generating Traffic - from fresh fishing pools of new prospects
2 - Boost Social Buzz - with social sharing links
3 - Improve Page Rank - which also helps Search Engine Rank
4 - Generate Backlinks - to help deliver fresh traffic day after day
As a rule, I tend to use a combination of both free and paid press release distribution services. For tip sheets and other evergreen type of content that I want to simply generate online links for, I use free services. For timely news, product launches or other type of content that is more worthy of being picked up by the media, I distribute through a paid service that has broader reach and is followed by reporters and media folks.
There are many good options for paid press release distribution, but my favorite at this time is http://www.24-7pressrelease.com. They have services that start as low as $49 and include great online visibility and social media tools. On the higher end, they include distribution through PR Newswire - one of the most respected old school wire services with a Page Rank of 8.
For free services, these are the ones that recommend.
There are also a few content sharing sites that are not actually press release distribution services per se, but can be used to help get your message in front of more eyeballs and new audiences.
As with most of the marketing strategies and tactics that I recommend, dabbling doesn't work. So, if you do choose to use Press Releases in your ongoing plan, consistency (at least one release per month or more) will be key to gaining the return on your time and financial investment.