When you connect the modem to the PC, the ISP see's the MAC address of the NIC in the PC. We added the cloning feature as a way to "trick" the ISP in to thinking that the same PC was still connected. This feature isn't really needed any longer, as MOST ISP's allow the use of routers. However the cloning feature can be useful if the modem is already bound to a PC's MAC and power-cycling is not an option, like with Time Warner's VoIP where the modem has a battery in it.