Change the 5V voltage source to vary the effective drain-source resistance. As long as the drain-source voltage is kept small the MOSFET acts as a relatively ideal resistance. Try and see what happens when you increase the drain-source voltage amplitude and also when you make the control voltage small.
This circuit technique can be used to make voltage controlled filters, amplifiers, attenuators and oscillators. The 6581 SID sound chip used in the Commodore 64 computer used this in its digitally controllable filter section with the help of DACs. The associated circuit nonlinearities (at high amplitude) gives the SID chip filter a characteristic audio distortion considered by some to be desirable.
https://en.m.wikipedia.org/wiki/MOS_Technology_SID
http://anorien.csc.warwick.ac.uk/mirrors/oakvalley/soasc/soasc_mp3/MUSICIANS/T/Tel_Jeroen/Cybernoid_T01.sid_MOS6581R4.mp3
