"The southern of these two army groups - in the center of the whole front - will have the task of breaking out the area around and to the north of Warsaw with exceptionally strong armor and motorized formations and of destroying the enemy forces in White Russia. This will create a situation which will enable strong formations of mobile troops to swing north; such formations will then co-operate with the northern army group - advancing from East Prussia in the general direction of Leningrad - in destroying the enemy forces in the area of the Baltic states. Only after the accomplishment of these offensive operations, which must be followed by the capture of Leningrad and Kronstadt, are further offensive operations to be initiates with the objective of occupying the important center of communications and of armaments manufacture, Moscow.
Only a surprisingly rapid collapse of the Russian ability to resist could justify an attempt to achieve both objectives simultaneously."-Führer Directive 21

The plan indicates that the first goal of the operation was to capture Leningrad and Kronstadt, then Moscow. It also points out that Moscow and Leningrad was not to be attacked simultaneously without the "surprisingly rapid collapse of the Russian ability to resist". The capitulation of Ukraine was not the priorty of the operation, and the goal of Army Group South was merely to tie up the enemy forces on its sector of the front.
Army Group South did not have any trouble doing what was assigned to it. The Red Army in that area was not a threat after the Battle of Uman, in which it had lost most of its tanks.
So the decision to move south was completely unnessercery.