When you penetrate the enemy lines with a sufficient force, you have your back to your own troops on the other flank.
You split their forces partly to disrupt communication between the two halves. But your communication and supplies remain single-focused.
Not only that, but if you penetrate the lines deep enough, you can cut off their retreat.
That'* not really what that term means anyway. Divide and conquer means to divide a team or disrupt them in such a way that you can beat them. The Oakland Raiders for example, are already divided. So just go conquer.