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Your question is about how to manufactur the mosfets or why implement Continuos source current using mosfets
If you see a datasheet of a device like PLL, logic gates etc, ine each one you can see current sources to bias a circuit, another use is active loads.
The differential pair uses a cuerrent source in the emiters to improve the diferential gain.
The current sources are a very important, usefult, and very used in electronics I can say that is the most used of all.
The importance depends on application. Capacitance will influence the switch speed.
Also Cds will affect frequency compensation if MOSFET is used as amplifier.
Source current will decide its driving capability.