There are so many great reasons to block your crocheting or knitting. However, a picture is worth a thousand words, so let me show you why you should consider blocking!
Here I did a swatch of simple chevron lace in Blue Bonnet Baby Wool. As you can see, there were quite a few problems with the swatch. First, the swatch was rolling, despite my best efforts to flatten it; this is because the lace pattern is based in stockinette stitch, which tends to roll. Second, the yarnovers for my lace are barely visible. Third, my stitches are a bit uneven; you can see that the ssks on the right side of the lace pattern are tighter than my k2togs on the left. Finally, I knew that this swatch wouldn't give me an accurate gauge. Even if I didn't block the garment I was swatching for, it will be worn and washed, which will change its gauge.
This is the same swatch after wet blocking. As you can see, the swatch is now flat with even, visible stitches. My gauge also changed considerably, going from five stitches per inch before blocking to four stitches per inch after blocking.
Want to learn more about blocking? Click here for more information on how to block.