This is called a "CHOKE" it uses an inductive magnetic field to remove noise on the cable called "Ripple". Its best to keep it there. The process of converting AC to DC through step down transformer gives you say 15 volts AC to bridge rectifier ( 4 diodes ) that cuts the sine wave into a half sine to remove the alternating from + to - and - to +, so you have a pulsating - to + direct current flow, then by use of a group of capacitors the 60 hz half sine if USA or 50hz half sine wave if europe is then converted into an almost constant DC that is linear with no half sine voltage with very minimal voltage over time fluctuation. The choke is used to protect the PC as well as the power supply from noise called ripple as well as to surpress a quick spike.
