While you might have the time to write your own press releases, you probably don't have the time or knowledge to hunt down the proper publications to contact or to make follow-up calls. An efficient, experienced PR agency should have boilerplate press releases into which you can plug your company
Not every PR agency is helpful for emerging companies, however. Some agencies deal only with large, established clients, while others are too inexperienced to add much value. Before you hire a PR agency, make sure that they like dealing with growth companies, that they train their younger team members well (there is nothing that reporters hate more than a flack who knows nothing about the publication or the company they are touting) and that the agency has experience in your industry.