Because it's handy to see if the item is actually made to fit somebody who looks like you.
Having been very small and very large, I knew that the smaller sizes would always fit, but the larger ones wouldn't, as the additional fat meant that my size was often at variance with the manufacturer's idea of where was larger - for example, a size 24 might have a very narrow front but three miles of room at the back or end exactly at my widest point, meaning I'd need a different size or item to be comfortable. Or, as in the case of knickers, that the band would rest exactly against my section scar and the loose skin from losing weight and cause friction burns compared to one with a higher waist.