Reverse-polarity protection P-channel MOSFET
Hard to definitively get into the head of the designer, but I'd suspect it's to overcome the Zener's capacitance and turn on faster.  If you picture a small capacitor in the place of the Zener, it would work along with the resistor to cause the output to ramp up at a constant rate to full voltage.  If faster on times are needed, the capacitor to ground can overwhelm the Zener capacitance and provide a quick turn on.

Zeners have characteristic curves; you don't have to drive them at full Zener current, but you should choose a drive current that provides a reliable voltage drop over your environmental conditions.  That current is regulated with the resistor.