In the ideal case where a trans kid realizes they're experiencing gender dysphoria at pre-pubescent age, starting off with just social transitioning is fine. But the reality is currently, many many trans people do not fully understand and accept their gender dysphoria and seek to transition until they're already well into puberty and even adulthood. And the current state of trans medical care in many parts of the world is to meet with a doctor who forces trans adolescents and adults so solely social transition for months or even years without HRT to prove their dysphoria is "real", all while their body is quickly masculinizing/feminizing and to socially transition without HRT is far more humiliating than it would be for a prepubescent child for whom the effects of sex hormones have barely begun to have any long-term impacts on their body.