If you listen to public radio the PDs make decisions about what and when programs get on the air. They do audience research and analysis to see what programs listeners like and dislike. They help with training to make the on-air hosts better. If you send hate mail or make a phone call they are often on the receiving end of your communication.
To learn more about PRPD and how program directors make radio better visit their website at or read the PRPD Blog.