if there is no insulation then there will be only conduction between gate and the body..... but since there is a insulation the body acts as one plate of a capacitor and the gate the other and hence the charge distribution is reciprocated ultimately forming the channel..........
Only by this insulator , v r getting very high input impedance and gate current Ig=0... If u r not having insulator , u cant even say it is a voltage controlled device..
Hope u get the points.
All the best