So, if the first draft is putting the clay on the wheel, then rewriting is where you shape that amorphous glob into a bowl or a vase.
And, here's a for instance for you...
I have two characters, and they interact. Subconsciously, I clearly wanted there to be some kind of romantic relationship to exist, based upon the descriptions and the way they act. It's during the rewrite that I can go back and add in and flesh out that relationship, adding depth and subtext to their interactions.
The subconscious surprised me all the time. It's way smarter than my conscious brain.