This quote has been attributed to Lenin by Norman Mailer in his novel of the CIA, Harlot's Ghost (although it is not known if he bothered to attribute it himself).

The essence of its meaning is, "Who profits?"

Mailer uses it as the mantra of his doomed Intelligence man as he re-reads his memoirs, trying to decide if his mentor (loosely based on James Jesus Angleton) was actually a Soviet mole like Kim Philby.

No conspiracy theory in the history of the world is complete until you can answer this question. It is similar to the principle of "follow the money", but must be taken to include all profit -- power, money, status, employment, representation, influence, you name it.