The purpose of using HCG is to restore testicular mass quickly. The biggest problem with recovery is slow recovery of testicular mass. Clomid and Nolvadex do nothing to quickly stimulate testicular mass. LH recovery is fairly swift, but what good is it if the testes' mass is diminished. So, again: HCG is used to quickly restore testicular mass. Nolvadex is sued as an antiestrogen and as a stimulatior of LH release from the pituitary. Llewellyn et al have demonstrated that Nolvadex seems to be a better LH stimulant and better antiestrogen than Clomid. Nothing stimualtes testicular mass quickly except for HCG.
An analogy: You place a 40-watt bulb into a light socket. You flip the switch and notice the amount of light given off. Now, replace the 40-watt bulb with a 100-watt bulb. Flip the switch and note the amount of light given off. The 40-watt bulb represents your testes after an AAS cycle and the 100-watt bulb represent your testes after using HCG/Nolvadex. Note that we haven't change the amount of electricity flowing through the light.