why is the feedback or gate resistance RG necessary?
The gate of a MOSFET is floating. There will be a bit of leakage (typically nA or less level at room temperature), and there is capacitance between the gate and drain and gate and source. So applying a voltage to the circuit without Rg will result in the gate taking on some intermediate voltage and slowly drifting up or down as the leakage current charges the capacitance (and the Miller capacitance affects the gate voltage once the drain voltage starts to move). 

To bias the MOSFET into the active region so it will work as an amplifier, in this case, you want the gate to follow the drain (at low frequencies approaching DC) and the gate to follow the input voltage for higher frequencies, which is accomplished by using a coupling capacitor. 

So a high value of Rg does not affect the input voltage too much but allows the bias to settle out in a reasonable length of time. For this we want the maximum leakage current multiplied by Rg to be a low voltage, and we want the coupling capacitor plus MOSFET capacitance including Miller capacitance multiplied by Rg to be a reasonable length of time for start-up. For example, if Rg is 1uF and the coupling capacitor is 1uF it would take a few seconds for the amplifier to settle down.