While corsets and dresses used to go hand in hand on every occasion, the change in fashion over time has meant that most women will rarely don a corset and dress in their lifetimes. One of the few times that modern women will wear this combo is at their wedding in the form of corset dresses.
Unlike the clothing of the past, corsets and dresses are rarely worn as separate pieces. Most wedding dresses have corsets built directly into them now in the form of a boned and laced top that will usually not require any undergarments to be worn underneath. This is most common in strapless wedding gowns, or gowns that use delicate illusion necklines that don't have a lot of strength in the material.
That isn't to say you can't find a wedding dress that doesn't have a corset top. In fact, there have been many designers that are now making filmy, sheer wedding dresses that have little to no lining, let alone a corset. Of course, these dresses then require you to purchase other undergarments to wear underneath or you run the risk of ending a bit 'overexposed' on your big day.
So what are some of the benefits of corset dresses and wedding dresses with corset tops?
For one, they're simpler.
As explained above, the custom fitting and structuring required to make a corset dress fit means that it will support your bust, lift and separate and trim your waist all without the need for other undergarments. Of course, this does mean having your wedding dress tailored to fit your body, but this is a practice so common that most brides already have it built into their budget.
Second, corset dresses can give you a lovely silhouette in no time at all.
Wedding dresses with corsets built into them function much the same way as traditional corsets. They use boning and lacing to shape a woman's body in specific areas, usually with the waist being nipped in to create the illusion of an hourglass shape. This can be great for women who worry that their waist-to-hip ratio isn't as dramatic as they would like it.
As for downsides, corset dresses are more rigid, which may mean that you will find it uncomfortable after a few hours. This is especially true if you're not used to corsets. Corseted wedding dresses are also more formal and traditional and may look out of place if your wedding has a hippie/boho vibe.