It is usually done when the sticker is slightly oversized and they're afraid putting it in the normal holder will damage it. I believe the plastic sleeve is supposed to make it harder for it to slide around. I'm not a big fan of how it looks, but my Bum Chex got that treatment:
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Nice Bum Chex! My Choke Wagon is off centered so much that you see the copyright for the card above it. I guess all of them on the rows above and below that Choke Wagon must've had the same problem.
