I started wearing a bra a couple of weeks after I started HRT, when my nipples were beginning to feel very tender. I used a sports bra for comfort under shirts when exercising, and for concealment when I was out in public, before I went full-time.
Now I wear a bra to make my nipples less obvious under a T-shirt, and for vanity when out in public. Now it's a padded bra to bring my less-than-AA boobs up to a semi-respectable A. I know that there are cis-women with smaller boobs than mine, but I need some figure to look more plausibly female.