Really? I see several: It puts violence into the minds of the people and could have been thought to help rally support for more control/intrusion*. It could have been designed to take the focus off of the gun-ban talks. It could be to get people to clamor for 'protection' and institute more police-state tactics. Remember Benghazi and Fast & Furious, these people have no problems sacrificing lives for their agenda.
* - This includes the possibility of having the government involved in all sales/business-transaction.
The only thing that has occurred to me is that they plan to use this to establish a pattern of violating Posse Comitatus, or perhaps to roll out a DHS program to set up road side (state) border checkpoints between the states themselves whereby people/goods entering a State would be subjected to profiling and vehicle search?
That's another possibility.
So, in effect, what we’re talking about is “conditioning”.