To a potential employee reviewing an actual job description is gold. It's like having all the answers to the test before you get asked a single question. Imagine going to an interview knowing all the primary responsibilities of a position and being able to tailor your every response exactly a company's particular need. You would be an interviewing machine - unstoppable - able to land any job, anywhere, at any time.
The next time you are in the hunt for new employment - do a little investigative work. Call up the human resource department and see if you can get your hands on a job description. If they say no (just as you might expect), Google or Bing your desired position and see what other companies are saying about the position. Check out the many employment websites and see if you can put a few more pieces together. You can also try searching for your desired position with the words "job description" after it and you will be amazed at the amount of data you will find. The more information you have, the more prepared you will be.