I know this is about trading card games and not board games, but I would say they’re very close in principle, considering the complexity of games like Magic. I recently discovered the mysterious world of “grading”, in which services like PSA will grade your card as to add some felt value to the card itself, because of its rarity and how near to mint it is.
We’re talking about services that will grade the same card differently three times if you send it back, because of course they have no practical way of having an absolute meter for every possible imperfection.
In theory, a service like PSA should be useful as a neutral third party that makes an unbiased appraisal of a card that both a buyer and seller can agree on. In reality, they are absolutely a scam and provide almost nothing of value. You could determine the condition of a card yourself using just a magnifying glass and five minutes of your time. For whatever reason, they’ve convinced enough people that a professional grade is necessary for buying/selling cards.
You are correct that their grades are subjective. I’ve heard of people who will send the same card back multiple times to try to get a better grade, which can be worth it for very high-value cards.