They do something, but I think it's not correct in Minecraft. In real life it increases costs which means lowering that cost has a benefit to the bottom line (i.e. a well meaning employee can justify this to people measured by profits/costs/etc.).
However, in MC where is the profit and does a carbon credit actually encourage players to try to do something seems interesting thing to see.
Well, one could make them arbitrarily unfair. Purchasing a carbon credit could reduce a player's own emission score but increase a random opponent's score by the same amount.
I'd prefer buying carbon offsets doing exactly nothing as in real life. It is a cheat that's not benefiting anyone.