I think MAC is more of switching and IP is more of decoding. In other words during network communication if we had only used IP then each time packets are sent from source to destination, the intermediate routers would need to decode packet and check IP address to verify whether it is the intended recipient each time packets travel through it, leading to more processing time. As opposed to this MAC address is at hardware level and hence it would be faster to check MAC address.
I may be entirely inacurate. Please comment your thoughts on this.