It is true that each device has a unique MAC address allocated to it by its manufacturer, but there is no proper standard for allocation of MAC the address in the industry. That means that every manufacturing company follows different standards for allocation of MAC addresses and hence it might be possible that by chance two different devices have the same MAC address and if they are on the same network, it will be very difficult to identify them uniquely and therefore we require, IP address to identify a device uniquely on a network.