When is a ethernet-to-thunderbolt cable necessary?
The main reason for Apple to develop the Ethernet-to-thunderbolt cable is because the Macbook Pro retina line is too thin to fit the ethernet Port, thunderbolt is thin enough to fit. If you have a MacBook Pro retina you have 2 choses:


Use Wifi
Buy the Ethernet-to-thunderbolt cable to use cabled internet on you Macbook Pro retina


The only reason: People who don't have wifi, don't want to use it, don't have a fast rooter,... 

If you have a normal unibody macbook pro there is no reason to buy the cable. :)