HCG (Human Chorionic Gonadotropin) can be used for male hypogonadism, which is a condition where the testicles don't produce enough testosterone. HCG helps by stimulating the testicles to produce more testosterone, potentially mitigating the effects of hypogonadism and supporting fertility. 
Here's a more detailed explanation:
Benefits of using HCG with TRT:
By supplementing TRT with HCG, men can maintain testicular size and volume, and sperm production, which can be important for those who want to preserve fertility. 
When HCG is used:
HCG is particularly useful for men who want to maintain fertility while on testosterone replacement therapy. It can also be used in cases of secondary hypogonadism, where the pituitary gland isn't producing enough LH. 
How HCG works:
HCG acts like the luteinizing hormone (LH), which is produced by the pituitary gland and stimulates the testicles to produce testosterone. When a man is on testosterone replacement therapy (TRT), his body can sometimes stop producing LH, leading to reduced testosterone production and potential shrinkage of the testicles. HCG helps bypass this and allows the testicles to continue producing testosterone.