As said in the following text (by Tietze-Schenk), this is a method
for simulation to keep the outputs (of a fully differential amplifier) at their required DC operation points, without deteriorating their ac behavior too much because of the huge inductances. In reality this is achieved by a
CMFB circuit, which compares the output voltages with a DC reference voltage and controls the current sources accordingly.
For simulation purpose the former method is simpler, as it needs only 2 inductance and the DC reference voltage source.