Since this is a hardware address is a unique number, this is a very easy way for the NID to indentify different computers on the network. Along with network troubleshooting and monitoring the NID assigns your computer its IP address based on this hardware address. If the hardware addresses are not unique, there is no way to distinguish between two stations. Devices on the network watch network traffic and look for their own hardware address in each packet to determine whether they should decode it or not.
Special circumstances exist for broadcasting to every device.
Each manufacturer of Ethernet devices applies for a certain range of hardware addresses they can use. The first three bytes of the address determine the manufacturer.