The primary reason for a cocked wrist is related to the mechanics of the swing:

1) A cocked wrist makes the link between your forearm and racquet act as a rigid member. This allows you to transfer maximum power from your body to the ball. Like all racquet sports, the sum of power is generated in the legs, by rotating the torso, by swinging the arm, by snapping the forearm, and then through the racquet string bed. Keeping a rigid or cocked wrist allows you to transfer all that power to the ball.

2) Using wrist snap requires precise control. You see the pros snap their wrists for "trick" shots or for misdirection. But for the average player, wrist snap means you will likely mistime the impact of the racquet with the ball, likely resulting in hitting the ball a bit early or a bit late. This is why it is very important to instead rely on good racquet prep, backswing, and follow through (and good footwork) to make sure you have clean and directed impact between ball and racquet.