The sticker is ment to increase selling price. Not all grades are equal. It's someone telling buyer that this book is really a good looking book in case buyer is unable to do that him/herself. The jokes abound because many (most?) feel it's not necessary and it's just another hand out to make money on slabbed books. How many folks can make money on (your) book?: Pre screen for pressing, Pressing, slabbing, sticker saying it's a really pretty book, auction buyer fees, Fed ex/UPS/Mail, state taxes, and then the gang comes around wondering/suggesting if you should reslab it (for a bump - ostensibly meaning more vaulable $$-wise) and start the process begins all over.
In a way it's sorta disheartening.