Stays (a stiff corset) were essential garments in the fashionable woman’s wardrobe throughout the 17th century. Some sort of stiffening of a woman’s gown had been part of dress construction since the early 16th century. Sometimes it was added to the outer bodice; sometimes it was in the form of separate stays worn under the gown. Originally the stiffening served the purpose of preventing the expensive and elaborately decorated fabric of the gown from wrinkling. However, because stays could mould the female torso, they became essential for producing whatever shape was considered fashionable.
Nope! Stays are not just “a stiff corset,” they are stays. A predecessor to the corset, yes. Usually more fully boned than a corset, yes. Not, however, a variant on the corset. The corset only comes along after th end of the 18th c, when you’re seeing more lightly-boned stays, and the term “corset” comes out of France to define that different style.
Also, it’s not to keep fabric from wrinkling. It is a bra. Women needed supportive undergarments (obviously, this is a super Western view, but it’s a western garment) hence, stays. Stays actually. don’t help you achieve any kind of nipping in of the waist - since they’re fully boned, they will actually add inches to your waistline. Yes, there’s definitely a look of conical torsos going on, but that’s not the stays conforming to fashion - that’s the fashion going off of the shape that stays give you.
We also tend to forget the lower classes in these discussions of fashion, and be assured they were wearing stays as well. Churches that were donating to poor women who couldn’t afford clothes made stays their first priority. While movies like Pirates and such make people think that women thought stays were oppressive and often went without them, this was really not the case.
Stays (properly fitted) are comfortable. They support your back, your breasts, and help your posture.