The pull-down resistors are to keep the logic level going into the gate at ground until the button is pushed.
IC's are VERYY sensitive to ESD (Electrostatic) and if you leave the inputs 'floating' (not conected) they will pick up signals from a range of things...
and you dont tie the input straight to ground to keep it at logic 0 because when when you press the button you will make a short circuit.
The resistor acts as a load.
It is a high value resistor to heavily limit current, but the IC will still pick up the logic levels.
You can change the resistor value to 10K if you want.