So that's why. I never knew that.Because when you purchase certain albums from iTunes you get the album artwork booklet as a pdf, but when you try to open it from your iTunes library it pops open preview.
I organize my music by seasons. "2008 3~4: Penryn iMacs," "2008 5~6: WWDC 2008," etc. I have to type those into the Album text box because there aren't any tags.So I can easily make a dance mix, happy mix, driving mix, etc.
And one more feature relating to point 15: Twin a music video or another video but keep only the soundtrack. So I have a music video and its music in two "separate" files in the iTunes Library (but actually one file in the Finder).