in the US it is mandatory; however it all relates to the US Mail. The used-to-be-normal way these bills are paid.

At the statement close date, the credit card issuer gathers up the month's transactions (this doesn't take 0 time, though it's a lot faster than it used to be), prints a statement, and mails it to your house. 1-5 days later you get it.

Most people would let bills pile up for 10-15 days then sit down and handle them all in a batch, writing a bunch of checks. 1-5 days for the mail to carry them back (you knew the time; you knew Virgina was 1-2 days and California 4-5). That is why the grace period is ~20-25 days.

Auto-pay makes all that seem stupid. But on the other hand, if there wasn't a significant time lag between credit card charge and EFT debit, it would defeat the point of the credit card, it would just be a debit card and those are already a thing.

And by the way, I've logged annual membership payments for a nonprofit and we capture the check numbers, so I can see how many checks they write in a year. A lot of people, still, write hundreds of checks a year, so they are not using auto-pay.

If there wasn't a grace period, you'd never be able to pay off your card.

Otherwise in the 25 days, your $2000 balance would accumulate $10 of interest, and your "payment in full" would only settle the $2000, not the $10. So the next bill would be for $10, which accumulates 20 cents interest. The third bill would be for 20 cents, then the fourth bill for a penny, unless they have a minimum interest payment, then the chain would never end. There'd be one more bill at $0 just to show that you cleared it. It would be bad for the credit card issuer also, because it would annoy the daylights out of customers, and mailing a credit card bill costs them $1-2. So the credit issuer has a big incentive to provide that grace period also, at least for bills that would leave small residual amounts.

in the US it is mandatory; however it all relates to the US Mail. The used-to-be-normal way these bills are paid.

At the statement close date, the credit card issuer gathers up the month's transactions (this doesn't take 0 time, though it's a lot faster than it used to be), prints a statement, and mails it to your house. 1-5 days later you get it.

Most people would let bills pile up for 10-15 days then sit down and handle them all in a batch, writing a bunch of checks. 1-5 days for the mail to carry them back (you knew the time; you knew Virgina was 1-2 days and California 4-5). That is why the grace period is ~20-25 days.

Auto-pay makes all that seem stupid. But on the other hand, if there wasn't a significant time lag between credit card charge and EFT debit, it would defeat the point of the credit card, it would just be a debit card and those are already a thing.

And by the way, I've logged annual membership payments for a nonprofit and we capture the check numbers, so I can see how many checks they write in a year. A lot of people, still, write hundreds of checks a year, so they are not using auto-pay.

If there wasn't a grace period, it would cause an annoying loop.

Otherwise in the 25 days, your $2000 balance would accumulate $10 of interest, and your "payment in full" would only settle the $2000, not the $10. So the next bill would be for $10, which accumulates 20 cents interest. The third bill would be for 20 cents, then the fourth bill for a penny. It would be bad for the credit card issuer also, because it would annoy the daylights out of customers, and mailing a credit card bill costs them $1-2. So the credit issuer has incentive to provide that grace period also, at least for bills that would leave small residual amounts.