While I don't condone it, I see this practice as a way to avoid having someone drive up the price of a card that you need for your set. Much as we hate to admit it, there are some guys out there who would take advantage of this situation [dealers??] and cause you to pay a lot more to complete your set.
It's a 2 way street. If you are like me and have a complete set with very high grades, you want to display it, in hopes it goes up. As with collecting its an investment. Even if you have no intension of selling it, you may never know when you may need to do that if hard times come along. So you want to get the highest value, if possible. I collect star trek deep space 9 and enterprise autographed cards and have the highest grades that are currently out there and most are 8-10s anyhow. (until someone gets one higher)
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Always looking for 1957 Topps BB in PSA 9!
I also have many highly ranked sets that I do show and include scans as well.