They are "shape ware." People wear them for different reasons. For example, people who are not heavy wear them to smooth out their bodies to give that really smooth appearance when wearing certain articles of clothes. Such as a tight, form fighting dress. It gives a nice look to your body.
Now if you have a few extra pounds, say some bumps, etc., it does the same thing. And you look and feel better.
Now they call them spanx, but they are just a new version of girdles. And they make them for men too. Yes, for men. In tee shirts and they are really hot sellers!
The worst and most uncomfortable item of under clothing ever. Once you have pulled yourself into it, it rolls down and cuts into you and in actual fact doesn't make you look that different. I've worn it twice under really tight clothing and took it off after an hour.