When a card has a stored value and the register will deduct that amount from the balance due, leaving you to pay the difference. So you don't need to know the balance on the card and it won't get declined because the balance on the card cannot cover the entire purchase.

For example, if the total is $15 and you swipe a card that has a balance of $1.05, the register will deduct the $1.05 from the amount due, leaving you to pay $13.95. Without auto-drain, the transaction would be declined (unless you know the amount on the card and the cashier runs the card for that exact amount).