If you're building an Internet-connected project, you might find that one annoying aspect of most Arduino compatible Ethernet devices is that you have to set the MAC address manually in the sketch. This is really frustrating, because every device needs a unique MAC address so you'll often find yourself modifying the sketch for every device you want to build. Even worse, you have to keep track of the addresses so that you don't re-use the same address and cause network problems.
This module solves the problem. With a simple I2C interface, it's pre-programmed with a unique MAC address that your sketch can load at startup using just a few lines of code.
Our own Ethernet enabled boards such as the EtherTen and EtherMega already include this same chip so you don't need the module on our boards, but for just $5 it can be a life-saver if you're using Ethernet boards from another supplier.