These hackers are like most in that they wanted to see how much data that they could steal but they didn't necessarily want to take that information now. If a cyberwar were to ever erupt then it is important to narrow your focus to your top priorities. It looks as if this hacking activity was part of the prioritization process.

The intruders want to explore the depth and breadth of each system that they were able to enter. This gives them a better understanding of the interlinkings of the various systems and narrow the focus to the weakest protective measures currently in place.

A breach this large and invasive requires the cyber experts to direct their efforts to defensive measures. This reduces the resources that are dedicated to the activities normally required for development of offensive cyber capabilities.

Lastly, there are occasions when you want the opponent to know that you are both capable and willing to conduct cyber attacks of a magnitude that have just been witnessed. This disrupts activities by the defender and when made pubic can shake the confidence of the public at large.