It prevents accidental turn on of the MOSFET by external noise usually at startup when the gate is floating. The MOSFET may sometimes turn on with a floating gate because of the internal drain to gate "Miller" capacitance. A gate to source resistor acts as a pull-down to ensure a low level for the MOSFET. I have had MOSFETs blowing up in high voltage circuits, without the resistor in place. In most of the commercial power supplies / inverters I have seen, there is a 1k resistor used.
A similar experience is narrated in Sanajaya Maniktala's "Switching Power Supplies A to Z". This is also talked of in Raymond Mack's "Switching Power Supplies Demystified".
Hope this helps.
Tahmid.