Ever wondered why there is a small 'sample' cloth included when you buy a new piece of clothing? It's pretty common with shirts and trousers. It's not to sew torn patches up, like I used to think earlier.
Well, they are given so that you can know the nature of the textile material used, so that you don't wash it in a way that can potentially ruin your entire dress. You can check it by washing the sample using a washing powder or bleach to know if it'll shrink or colour bleed.