When you release your string, the arrow remains on the string until the string reaches the brace height, more or less. Until that point, the arrow will move towards the part of the string that was pulled the furthest, which is typically where your middle string finger is.
That downward movement on the string, however slight and however quickly, will cause your arrow to bounce off the rest/shelf, making tuning nearly impossible. A second nock, below the arrow, prevents that unwanted slide down the string.
It's important to leave a small (1/16") gap between a nocked arrow and the bottom nock to prevent the nock being pinched at full draw.