In most cases, such a circuit is used as an amplifier vor signal voltages. Thus, it is desirable that the input resistance of the amplifier stage is large in comparison to the source resistance of the signal source. The total input resistance is mainly determined by Rg - thus, Rg should be large. But keep in mind that it is not the real value of Rg that is to be considered. Due to the Miller the effective value of Rg is reduced.
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