Your billing address helps the credit card company confirm that you are who you say you are.

THSN will reject your order if the credit card number is wrong, or if your address does not match the address the credit card sends its bills to.

At THSN you must enter a billing address, and for many that must also be the shipping address, so we can be sure you are not a criminal who has stolen a card and wants stuff sent direct to a motel room.