Purpose of input resistor in ADC circuit
It's possible that the author of this schematic fragment intended the 1M resistor to be representative of the ADC's input impedance, rather than a physical component. In that case, the resistor is not a discrete part you add, but aids calculating things such as how much the ADC affects the voltage at that node.

If it was intended as a physical component, it's a bad idea. The ADC already has a high input impedance - as indicated by the 1uA leakage you quoted - and a large series impedance will create error in the ADC. The node that the 1Mohm resistor is connected to is very low impedance in any case, so it's not necessary to add a series resistor here.