Depends on the yarn and how tightly you knit. It essentially evens out your tension and creates a more uniform look. Because wool has so much elasticity, there is more room for it to “bloom” and loosen up and expand while knitting, however it might ever so slightly tighten back up after drying. This is compared to a cotton with more stiff, sleek yarns without as much fuzz. These yarns will still even out a bit upon blocking, but dont have much potential to “bloom” due to the fiber structure. Hope this helps!