Every public statement that the POTUS makes is a pretext for an action which he wants to pursue, or a justification for an action already taken.

US publicly accuses Russia of hacking the DNC and states' elections infrastructure. 

I see some options here on the action in view: 

  1. Postponing the federal elections in November. Because Hillary might lose. Trump is apparently the wrong kind of New York liberal. 
  2. Nationalizing the election process under DHS. Because the prime directive is expansion of federal power over anything and everything. What tenth amendment?
  3. War with Russia. Because the petrodollar dynasty must survive. Russia selling way too much energy in currencies other than the USD. If a massive economic center of gravity is allowed to be established in the East, the West will collapse under the weight of tens of trillions of dollars in debt.  

To make this public announcement and then take no action would be an admission of weakness and impotence. The only utility in confirming these embarrassing suspictions would be as casus belli. But against who?