[This does not contradict what EMFields wrote above. This is just another point of view.]
The purpose of the pull-down resistor is to give the output a known state when the wiper is in the gap located at the North.
Have a look at the figures from p.2 in the datasheet.
- Assume that the wiper is at NW. The output would be around 90%.
- Now assume that the wiper is turning clockwise from NW. The output will be growing.
- Then the output will reach 100% when it reaches the point T1 on the circuit, which is at 358.25°.
- The output will stay at 100% as the wiper keeps going clockwise between 358.25° and 358.85° .
- At 358.85°, the wiper becomes an open circuit.
If there is no pull-down (or pull-up), the output would be floating. It would act like an antenna for interference an leakages. The reading is unstable. The measuring equipment may even see 2.5V because of leakages and think that the vane is pointing South while it's actually pointing North.
So, the pull-down gives the output a determined state when potentiometer is open circuit.
Why 470kΩ ?
I think, it's a ballpark number, and it's driven by a compromise.
On one hand, you'd like a strong pull-down to reduce the effects of interference. Strong pull-down means smaller resistance value.
On the other hand, the pull-down is in parallel with the lower leg of the potentiometer, and it's introducing non-linearity into the measurement. So, a weak pull-down is desired. Weak pull-down means larger resistance value.