1. I can "read" a book in the car without vomiting.
2. My kids can hear a book in the car without also hearing vomiting (Frodo usually drives, so I would be the one left to do the reading).
3. Having kids experience great books that are above their reading level, but not above their ability to comprehend.
4. Better than an in-car DVD. I know this can be controversial, but I much prefer audio books to movies in the car because we all get to hear books we might not normally pick up to read (for example, my kids heard The Perfect Storm a couple of years ago, and we grownups heard one of the best books ever written, The Tale of Despereaux). Plus, I can listen to great "children's" literature and say, "Well, I was only putting it on for the kids." (Although no one will believe me now, since admitting that The Tale of Despereaux is one of my favorite books.) Also, watching a movie is passive, but listening to a book exercies the imagination. And besides, have you ever tried to put an in-car DVD system in a Taurus?!
5. Entertainment. I have rarely enjoyed myself more than the time spent listening to Tim Curry read the Series of Unfortunate Events books.
6. Education... I am learning a great deal about the Greeks and Greek history (more than I ever learned in school) by listening to Sailing the Wine-Dark Sea. Also, the kids must ask me dozens of times what different words and turns of phrase mean while we are listening to books.
7. Unexpected entertainment. Yesterday, we were listening to an Arthur audio book in which Arthur is playing on a baseball team for the first time. During Arthur's first game, Arthur is in the outfield dreading having the ball come in his direction. "The pitcher throws the first pitch, and it's a ball." Primo immediately pipes up, "Of course it's a ball! What is it supposed to be?! A hockey puck?!" (Which brings us back to point 6 in which Primo received a lesson on the rules of baseball.)
A note: Although my above list implies that we listen to audio books only on long car trips, this is most certainly far from accurate. We listen to audio books almost constatly when we are in the car... running errands, going to church, going to a friend's house. We always keep an audio book with us in the car. We will listen to music on occasion (and rarely the radio), but we LOVE our books!