The unique objective is that the PC recognise the MAC address assigned to the device. (cmd, ipconfig /all,...).
If you do nothing with the device the PC won't recognize the MAC address as it will not see it. An inactive networking device is not visible on the network.
With an Arduino Ethernet Shield you can define any MAC address you like.
Please tell us what you want to achieve, we might help you more if you tell us more about your project.