And it also prevents interferance, Electro Magnetic Interference. Ferrite cores should ALWAYS be affixed to the power cord or whatever, since if it isnt it violates the EU laws and safety on EMI (Electro Magnetic Interference) and RFI (Radio Frequency Interferance)
Ferrite rings are to reduce rf(radio frequency) interference--these are built in to many connecting leads but can be added as extras to many devices which may be rf sensitive. Cables can be effective aerials and the performance of a number of electronic device may be affected if these high frequency signals are fed into these.
Apart from pc's and game consols ,hifi devices such as cd players are examples.