You have other glands within your endocrine system that require leutinizing hormone to operate properly. As exogenous testosterone (TRT) turns this off at the pituitary, these glands (adrenals, thyroid, etc) aren't able to work as intended. As a result, pregnenolone and other hormones start to falter. HCG mimics LH even while the pituitary is suppressed, allowing production of these hormones to continue.
I totally recommend it at least as a test to see if you feel a noticeable difference. Of course, this also keeps you fertile, and prevents atrophy of the testes; which many men take it for on top of the other benefits.
Be advised though, some are a little sensitive to the effects, so if you notice estradiol side effects - HCG doses may need to be reduced, or eliminated altogether if the sides outweigh the benefits.
My .02c