The core of any transformer is laminated so as to: reduce the energy loss due to eddy currents; make it light weight; make it robust and strong; increase the secondary voltage

Eddy currents are the loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction. Figure (a) shows eddy currents within solid iron transformer core. Making the core out of thin laminations (figure b) parallel to the magnetic field with insulation between them reduces the eddy currents and thus also reduces the energy loss due to eddy currents.