The use of corsets has changed from practical to fashionable. Older corsets from the Victorian era were made to be worn underneath a garment and were used for things such as supporting the back or waist training. Nowadays they aren’t as tightly laced and are mostly used for fashion purposes only.
Modern day corsets are usually not worn undergarments because they are made to look good and to be shown off. For example, the corset on my mood board by Monique Lhuillier with the embroidered flowers is a statement piece and therefore should not be hidden.