Why do companies choose PR?

Many people who hire a PR firm often do so without knowing what PR really is. For instance, recently I was in London and two PR practitioners said that their clients often thought that PR should boost sales. They also had the problem with clients wanting advertorials placed in publications rather than features and news. There clearly is a lot of confusion as to what PR is and how it should be used. Many clients often go to PR because they can't afford advertising and see PR as a cheaper alternative (which it is) but do not understand that it is also very different to advertising.