For online markets, a press release can serve two main purposes:
Journalists are looking for stories all the time, but they are bombarded by dozens of press releases daily. If you really want your press release to be read, you need to put yourself in their shoes and provide them with information they are looking for.
To illustrate this, we can use two extremes. Let's assume you just launched a new dating website, and you are writing an interesting heading for your press release:
Heading 1: "Company X Launches New Dating Website With Excellent Matching Services"
Good heading, but who cares?
Heading 2: "Company X Launches New Dating Website and Urges Paris Hilton To Be Its First Customer"
Well now, simply by tagging a celebrity name to the press release, you have caught the attention of many long enough to review your website and write about it. You even gave them a story to work on.
If your press release gets the notice of just a handful of journalists working for well-known news sources, then the articles they write about you will have a significant distribution that will generate you considerable name recognition and website traffic.
Furthermore, press releases, even the ones that do not get caught up in a media storm, are very likely to get archived by various news websites which means a permanent link to your website from high ranking web pages.