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It looks like an adaptive biasing circuit. This might be adavntages when your signal varies over a large range and hence if you have fixed bias your circuit might start to slew, now having adaptive bias slewing can be avoided.
Also, the current consumption depends on the signal level, hence useful for low power consumption.
It is self adaptive biasing circuit, where it controls the drain voltage of the amplifier, so that it could keep the drain voltage to ensure the tail transistor to be in saturation region always for linear gain amplification and as for the common mode feedback as well.