they keep the saddle from sliding, of your saddle is sliding and it fits your horse and your girth is tight, you might want to invest in one
The point of a breastplate is to make sure the saddle doesn't slip backward off the withers. If you don't tighten your girth enough and you don't have a breastplate that's a disaster just waiting to happen lol
It's to stop your saddle from falling back when you go over a jump
It's for if ur saddle slides back on ur horse. If u have trouble with ur saddle moving on ur horse.