Long story short, it works. HCG works by mimicking LH, the hormone that tells your testes to produce testosterone. When you add synthetic testosterone your body may stop producing LH, which can lead to testicular atrophy. Using HCG during your cycle helps keep the testes at their normal size.
For guys planning a PCT, incorporating HCG can make coming off easier by maintaining testicular function. Some choose to start HCG in the last 2-3 weeks of their cycle to jump start the testes before PCT. There's different ways to use HCG, but these are just some quick takes.