Being portable is ideal for business people, because they can listen to information while they are commuting. That way they are being productive even while waiting in traffic, or anywhere else they go.
Audiobooks can also act as a way to learn things or improve while you are asleep, through self hypnosis. It has been proven that a person can learn much better while they are asleep, and absorb the knowledge better than when they are awake. It is like subliminal mind control, but you are controlling the programming.
You would be surprised to know the amount of information available on audio.