Using press releases for site promotion encompasses two strategies which appear similar but are in practice very different.
The first strategy is to use press releases, press release sites, and press release distribution services to deliver your message, in mass, across the globe.
This strategy requires that you either write a press release or have one written, and that you then distribute it through sites like these:
The second strategy also involves writing and distributing press releases, but that's where the resemblance ends.
The problem with the mass distribution strategy is that the best avenues to promote your press release don't look for press releases at any of the high-volume press release sites. Those sites are spammed by every marketer on the Internet.
With targeted distribution, you first define who you want to publish your press release. Then you write a press release for them. Then you contact them personally and ask them to publish your press release. Then you listen to their feedback. You may end up completely rewriting the press release based upon that feedback.
The goal here is either to be published by the niche specific news outlets who will help you build targeted traffic, or to be published by the high-volume mainstream media reporters who are difficult to reach by automated methods. Most of these news outlets won't give you live links, but they will create interest in your site or product.
Mass Distribution is better for building links; Target Distribution is better for building buzz.