Gender-affirming care, also known as transition (transition what? Their sex!), is done in order to make trans people more comfortable in society, in their inner self (hormones affect mood, behavior, feelings, etc) and in their body by changing their sex. This is why they are prescribed sex hormones as these are what guides gene expression and decide sex-dependent characteristics. While some trans people would be content with just changing their physical appearance most of them seem to actually aim to change their sex.

Gender-affirming care and gender transition (as the results on Google will call it if you accidentally search "sex transition"), none of this changes your sex, most mammals (including humans) have the XY sex determination system, taking estrogens or other hormones doesn't make you lose your Y and gain an extra X, nor does it make you start producing ova, the only difference is how you look and sound, which (hopefully) makes you more comfortable with your gender, and that's why it's called "gender-affirming care" and "gender transition".