Lay sprinkler lines and start watering. I was surprised by this step as well. The goal is to encourage everything that still wants to grow to show up -- so we can kill it off. Since the soil was tilled and regraded, odds are that a lot of stray seeds have been tossed back into the soil. If we went ahead with laying sod, we would end up with weeds and non-grass trying to poke back through. So our landscaper takes this step to try to make sure we start with as clean a slate as possible. If the sod gets to take root and establishes itself, we'll have less work on our hands now and later since it's a self-sustaining thing. One of the best bets against weeds is maintaining a healthy, lush lawn.