What is he speaking of when Winston says I understand the how I do not understand the why

Winston was thinking about the photo of Jones, Aaronson and Rutherford. They were labeled "traitors" by the party. Winston knew that the confession of these men was written and re-written to suite whatever the Party wanted at the time. He understood how the party did this; it was infact part of Winston's job at the Ministry of Truth. Winston wonders why go through all this trouble to manipulate the past? People have long forgotten these men yet the party continues to alter records to reflect present circumstances. Winston will indeed discover why. Those who control the past control the future. Winston figures out that the party wants to control reality regardless of what really might have happened. If they can change the past , they can control the reality of the present and the future.