Why there is 3 phase mains input for this AC/DC power supply?
1kW is a high enough load that the balanced 3-phase circuit is an advantage. 

Another advantage is that the filter capacitors need be much smaller for 3-phase, since a the output of a full-wave 3-phase bridge has less than 15% p-p ripple without any filtering at all. That would generally mean lower cost and longer MTBF for the power supply.