Interest accumulates daily whenever someone carries a balance on a credit card.
So not only can sending in that payment the day a bill arrives protect people from being slapped with a costly late fee, it can save them money. And the bigger the balance, the more interest is saved by getting that payment in days or weeks early. Sending a half payment every two weeks results in 26 half payments, or 13 monthly payments a year. So paying this way gives a customer the equivalent of an extra monthly payment each year. And getting a portion of the payment in sooner knocks down some of that daily interest.