Using MAC (Media Access Control) address filtering on top of encryption is another obstacle you add to wireless security. Most routers provide this service. This feature only allows authorized computers on your wireless network. Each network interface has a unique MAC address assigned by the manufacturer.

IOW, having the encryption key is not enough to gain wireless access. Your computer's hardware address has to be specifically authorized (or spoofed) on your router to gain access.

Again, this is about making your system just a little harder to break into than your neighbors'systems.