I often was confused as to why a MAC address is required for a network node when we have a IP address. The MAC address is actually a hardware address that is assigned by the manufacturer and is gauranteed to be unique.
Basically the MAC address is used in the Data-link layer and the IP address is used in the Network layer. For e.g. Ethernet uses MAC address to find other hosts.
The ARP protocol does the mapping btw the MAC address and the IP address.