Why is a MOSFET in series with a power rail, with the gate connected to ground?
As Tom hints this is reverse voltage protection on the +12V line, however you may find it confusing for two reasons:


The MOSFET is used 'backwards' when it is 'on'. Not a problem for MOSFETs, they conduct equally well in either direction. This is because the body diode must be reverse-biased when it is blocking negative input on the +12 input pin.
The MOSFET symbol is wrong - the part is a p-channel MOSFET, but an n-channel symbol is used. Try redrawing it with a p-channel symbol that includes the body diode.