Above is a test swatch I made for a belly dance belt I am going to crochet. Not only does test swatching help you practice gauge (I found out that I have more tension in my stitches than in my rows hence the rectangular shape), but is a great way to test for bleeding. Recently I made a shawl that the rose color bled into the mint color. I went to the yarn shop and found out that:
1) I don't have to block with warm water, cool water reduces bleeding
2) Soaking the article in a solution of 8 parts water to 1 part vinegar should set the colors
3) Test swatches are a great way to make sure you don't use colors that won't work together before you spend all those hours making that handmade gift
Another benefit is finding out how your natural materials will react to washing. If you know a pattern will fit you in a medium, you wash your wool, and the water makes the wool shrink or loosen you can adjust accordingly. Great for garments especially.
