Two reasons---one is because the high output voltage solar panels require it. Regular charge controllers connect the solar panels directly to the batteries until the batteries are charged, then disconnect. This only works when the panel voltage produced is close to the battery voltage. MPPT controllers sample the battery and solar panel voltages and currents and determine the maximum power point, where the best power transfer occurs. This brings us to reason number two---MPPT controllers are up to 30% more efficient than conventional controllers, because of this constant monitoring and adjusting of the voltage/current (power.)
Grover--- Resident prognosticater. If you need specifics, contact me at firstname.lastname@example.org.