It is as much a scam as the cards themselves. Artificial scarcity designed to addict our brains to gambling.
The grading feeds on/into the main scam of the cards themselves. The whole cycle requires that we place "value" on their slabs of paper and grading helps make that value more concrete in your brain. The more value you see in the cards the more of them that you want to buy. More cards sold means more grading, so the cycle goes round.
Have fun playing the game, just be aware the whole thing is no different than going to the casino. They just hope the psychological effect of not gambling with money addicts extra people who wouldn't normally gamble with money (i.e. children or those morally opposed to gambling)... and the house always wins.