For example the official pretext for WWII was started as a false flag: https://en.wikipedia.org/wiki/Gleiwitz_incident US did it at the start of Vietnam War: https://en.wikipedia.org/wiki/Gulf_of_Tonkin_incident
We gain a lot from this. We can for example manufacture "Russian hysteria" - "Look we found a Russian rootkit on a DNC server". We can attack our allies and then make it look like the Chinese did it, and so on. It is immensely useful.
Implying a 'Russian rootkit' was planted in a false flag operation?
So the CIA pretended to be Russia helping to get Trump elected -- why?
Why do you think they might want to pretend to be someone else?
It can confuse the attribution so that trust is spoiled, so that energy is spend uselessly, so that another country takes the blame, for false flag attacks to justify other strategic moves.
Like it's the case with the NSA: http://www.networkworld.com/article/3137065/security/shadow-...