|image source: Funny signs|
The first corsets were stays, a 16th century invention that gave the torso a cone-like shape and pushed the breasts up, like this.
By the 19th century stays had been replaced with the corsets we associate with the Victorian era, an undergarment which created a tiny waist and hourglass figure. It went out of fashion by the 1920s. Although it can be associated with fetishes and BSDM, it has made a mainstream comeback--not as an undergarment, but as a top, much like a shirt.
Why did we adapt this piece of underwear for our everyday outerwear?
As someone who has worn corsets in public before, I have to scratch my head. The Victorians would be horrified--it'd be like women wearing just bras without some form of clothing (like a shirt) over it, such as Brenda Strong's character did in the Seinfeld episode "The Caddy." But the corset still has that association as an undergarment. Unless I wear a sweater or some other layering item over the corset, people think it's a camisole. Take away that covering item, however, and a girl is bound to get elevator eyes or catcalls. This despite the fact that Charlotte Russe has been selling corsets for at least four or five years, which means that the corset can't be that uncommon anymore.
I wonder if this is because the corset, in some way, is associated with sex. From watching enough modern movies set in 19th century London, the prostitutes mainly seem to be wearing corsets. At least any clear shots of Rachel McAdams in the new Sherlock Holmes trailer are the ones in which she is wearing a corset.