A corset was a must in the early fashion because it girds the torso and shapes it accordingly as defined by the fashionable silhouette of the days when women were more expected to bear a perfect figure. Some women wore corsets tighter than necessary and bought corsets with smaller waists. Corsets were then typically worn inside the garments. According to stories, some women who purchased an 18 or 20 inch waist corset ended up with broken ribs, having ribs removed, and causing fatal injury because of tight lacing. [Ouch, that’s gross] But I think it’s just exaggeration, lol.
Fact is, corsets serve one primary function: to give the body a "fashionable" shape. Thus, it is not a surprise that ancient corsets evolved into the modern era.