The main use for a bodybuilder are:

PCT use

Using steroids will suppress natural test production. After your cycle, natural test production will begin again on its own.  It is a very slow process. PCT will speed up the recovery process. It will not return your levels to normal on its own, but it will ensure you have enough testosterone for proper bodily function while your levels continue to naturally rise.

On Cycle use

Using steroids can cause testicular atrophy. By supplementing with HCG during steroid use, the individual can keep his testicles full. It can also lead to an easier road of recovery once the cycle has been discontinued. Keep in mind, if you are not a low testosterone patient HCG use on cycle must be regulated heavily and monitored closely.