Its a Leutenizing hormone mimetic.
LH signals the leydig cells in the testis to produce testosterone. Taking HCG offers a few benefits. It allows the leydig cells to maintain function while you are shut down. This is a good thing. It also helps backfill hormonal pathways that woud normally be shut down as well, another good thing. A couple things to remember is that you should take it at a low dose. Very high doses cause the leydig cells to become desensitized. This effect is more dose dependant than duration dependant so just keep doses on the lower end. I would say under 1000iu's/week would ensure that this does not occur. I really like the 250iu's/2x/week protocol.
You may need to keep an eye on your e2 as the added production of test will also increase aromatization but again , low dosing of HCG minimizes this concern as well.
Overall if taken properly it offers several physical and psychological benefits that make it a worthwhile addition to your trt protocol as well as to a cycle.
Hope that helps. If you have any other questions ask away. Much like yourself I dont venture lightly into taking more compounds so when I started using hcg years ago I did a bunch of research on it and its use so I have a decent grasp on what it is, what it does, and why it can be beneficial.