We have all signed up for a "free 7-day trial" only to forget to cancel before the billing date. With a discardable card, you can set a $1 limit or a one-time-use rule. When the trial ends, the merchant cannot charge you.

Platforms like PayPal Developer or BetterBugs provide dummy numbers that follow the Luhn algorithm (the checksum math used by real cards) but have no monetary value.