In Europe there are stricter standards for reducing harmonics on electronics (CE compliance). Alot of devices come with the donut which serves as a line choke. It is a line reactor that is saturated at the fundamental frequency(50-60hz), but reduces current flow at higher frequencies. It is just low pass filter.
You use a line chock on switching power supplies, which are rich in harmonics. The choke delays the current flow(with no power loss), like a shock absorber, to reduce harmonics. When the transistors in the power supply turn on there is a reduced inrush current. I understand a ballast in lighting does the same thing.