We seem to highly regard DEMO over anything else.

The majority seem to regard demo equipment more desirable than anything else, used. Could someone please tell me why? I know there is a warranty but I have never had a piece break down in the first 2 years .
I remember way to much time I spent at my local audio shop, as i remember they plunged in the equipment and unplugged it 6 months or so later, I guess tubes were unplugged every day So why is demo so great? I hope this question is not to boring for everyone!