Why does a three-piece suit have a vest/vest?

traditionally, suits and vests are not worn out, so it is natural to use relatively cheap lining on the back. However, there is a trend to wear only suits and vests. Don't do this on formal occasions. Dinner is usually suitable, but it is fashionable and exquisite. Make sure that your shirts and vests are slim and fashionable. The back of suits and vests looks good. Look at the vests of athletes in billiards games.