This is a great article on HCG
Prescribed HCG
HCG alone is ok, its called HCG mono therapy. The problem is most guys end up trying testosterone and once you do HCG is just not strong enough anymore.
The reason you want to do both is because testosterone is suppressive to the HPTA axis. Through the negative feedback loop your body reads high testosterone levels and tells the pituitary to stop sending out FSH and LH. So now your testes do not receive these two hormones. If you don't use it you lose it, that saying holds true here.
Your balls basically die, they become insensitive to LH and FSH hormones. This causes them to shrink, stop making sperm and progesterone. Most people don't realize this is the lubrication for your arteries.
This negative feedback loop also suppress other hormones in your body. Its really simple, you want to keep as many processes going as you can. Whether your on testosterone or not.
She is probably worried that you are trying to get jacked, in her mind taking HCG and testosterone together will put your testosterone levels too high and you be "doing steroids" or whatever she calls it.
Thats not true though, HCG is not very strong and adds little to the way of testosterone to the total amount in your bloodstream. When your doing TRT your just taking enough to keep them alive, not enough to keep them pumping out at high output.
Hope this makes sense.