We don't. We can communicate without IP addresses on a LAN, and there are protocols that do that. But if you already have a TCP/IP stack on the computer, it's wasteful. Now you have to maintain two logical communication protocols. Now you have to switch between them if you want to relay information out of the LAN. read more
up vote 5 down vote favorite Because MAC addresses are only usable across the local network segment, we use IP addresses to communicate with other segments via routers. read more
IP addresses are a part of layer 3 in OSI network model. For just switching purpose we do not need IP address but as mentioned by someone in the answer we need it when we perform Inter-VLAN communication because this needs a layer 3 device and as mentioned earlier when layer 3 device comes into the picture we need IP address. read more