That headphone jack is for more than just playing audio content. There is a built in Text to Speech engine in the touch and keyboard models. It has a bit of a robot sound and sometins mispronounces words, but you soon get by that as you listen to the storey. It also turns the pages while it reads so you don't have to look for your place when you go back to reading it yourself.
I have my Kindle keyboard reading to me through my car speakers with a simple adapter cable whenever I am on the road. Keeps me calm in heavy traffic and I don't feel like I'm wasting so much time just getting from point a to point b.
I have over 300 books on my kindle so far and haven't put a dent in the 4 gig memory yet.
Now that I can get kindle books from my local library, it really hits the mark.