Another comment gave you the gist, but it's also worth noting they can give you control over how much can be debited, how often it can be debited, etc. You can even use them to make budgets for yourself. The leading provider that I know of for this is https://privacy.com/

Haven't used one myself but iirc a credit card number is generated on the fly for each transaction (or maybe a set of transactions). So someone trying to charge you again will fail if it's no longer valid, and you can't correlate it easily with other data.