You would block cotton for the same reason you block other fibers: to even-out the stitches, smooth the edges so that seaming is easier, and make sure that the finished item is the right size. In short, to perfect your knitting.
However, if all the pieces are the right size and shape, you don't really have to block each one individually. Sew up the sweater and wet block it the way Maggie Righetti suggests in "Knitting in Plain English." Basically, you hand-wash the sweater in mild soap and water, rinse well, roll it in towels to get rid of most of the water, and lay it out on more towels. At this point, you can pat and prod it into exactly the shape you want. Then let it dry completely. You end up with a well-blocked sweater that is also clean and fresh -- free from the soil and other gunk that inevitably gets on the yarn as you're knitting it.