The pulldown is not strictly necessary but it prevents the MOSFET from being damaged should charge accumulate at its gate (easy to do due to electrostatic buildup). MOSFET gates can be damaged by as little as 20V while your accumulation of 1000V on your body of electrostatic charge is barely perceptible when you get "zapped". So it doesn't take much "stray charge" to destroy a MOSFET. The 47k resistor provides a path for that charge to leave the gate. When the Arduino is off or is in reset, the pin driving the gate is floating thus does not pull the gate neither high nor low, which is when bad things can happen (as they can while you are building the circuit, so put the resistor on first 
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Beat707: MIDI drum machine / sequencer / groove-box for Arduino