Audio Books and Hard Copies

So I have a question for anyone that listens to audio books, do you just listen to the audio book or do you also buy a hard copy or vice versa? I have a friend that buys the audio format of books that she already owns hard copies of and I think it's just a waste. If I buy a book no matter what format it's in I don't see a reason to buy it again. On a similar note if I start a series on audio I usually like to continue the audio version.