They don’t have to but it’s nice that new car audio does. Most portable players will use the MP3 format due to it’s compression. If you went to burn an audio CD to listen to in your car you might get 15 or more songs converted from this format. If you ever copied MP3 files directly to a CD you get about 10 times the number of songs on a CD or about 160. That’s nice for a long road trip.