The main reason for album art in my opinion

Imagine buying an album at the usual 10 to 15 pounds that you usually would pay and it coming in a plain plastic or cardboard sleeve with a blank disk with nothing more than a title crudely printed. I and anybody else would feel extremely ripped off and would not ever want to pay anything close to 10 pounds. This is the most important thing about album art and nice packaging it makes you feel as if your getting more than you actually are and it plays a trick on your subconscious tricking you into thinking its money well spent.
Alot of bands even today will release a normal version of the album and then either very shortly after or at the same time a special addition with fancy packaging at a much larger price with as little as 2 extra bonus tracks. Even though you get so little extra, i myself have being guilty of double album buying because of nice packaging and the promise of an extra few track or extra booklet full of art.
 The last time i committed this crime was with an album called blood drunk. There on the hmv shelf was the normal version at 12 pounds and the special addition at at 20 pounds. With the promise of a dvd and a fancy embossed cardboard sleeve that the album fits into i went with the special addition only to find that the dvd contained the same content as the cd only in surround sound.
My point is that the art on the album is basically a sales man and makes album art today and until cds completely die out very very relevant and shows that the job graphic designers and artists do on the album art is almost as important as the band writing good music for its release.