Original: A magnetic core can increase the inductance of a coil by a factor of several thousand , by increasing the magnetic field due to its higher magnetic permeability . However the magnetic properties of the core material cause several side effects which alter the behavior of the inductor and require special construction :
Simplified: If you use a more magnetic material as the core , you can get the magnetic field around the inductor to be pushed in towards the inductor , giving it better inductance .