What is the purpose of using MOSFET instead of free-wheeling diode in Buck topology?
http://www.digikey.co.uk/Web%20Export/Supplier%20Content/Semtech_600/PDF/Semtech_synchronous-vs-asynchronous-buck-regulators.pdf?redirected=1

Forward-biased diodes are not perfectly conductive; there is a voltage drop of 0.7V (0.3V for Schottky) across them. At high currents, this results in high power dissipation across the diode. High-current diodes may have longer recovery time too.

When the lower MOSFET is on, current flows through it rather than the body diode. The MOSFETs are selected for low Rdson (on resistance), so the minimum of energy is dissipated in the MOSFET.