What is EMC recommended circuit and is it necessary?
It's purpose is for reducing conducted RF emmissions for Electro-Magnetic Compliance (EMC) purposes.
Basically if you want to pass compliance testing (e.g. CE mark, UL, etc), it may help reduce noise levels conducted out of your device onto the mains or DC power input.
Whether or not it is necessary is for you to determine. But if you have the space, I'd include it.