When Daniel Ellsberg leaked the Pentagon Papers, his primary goal was changing U.S. policy in Vietnam. But he also had a "very important secondary objective" -- he hoped that Americans who read the documents would lose their tolerance for granting the executive branch the ability to act in secret. They'd see that the Vietnam War was enabled by that secrecy.
As we ponder whether the Obama Administration keeps too many secrets from Congress and the people, it's worth returning to a passage in a 1973 interview that Ellsberg did with Reason magazine (emphasis added):
Without the widespread willingness to allow the executive to keep secret the mass of information about its own operations and intentions, it wouldn't have been possible for the executive to steal away so much power from the Congress and the public and to free itself from the kinds of checks and balances that were intended in the Constitution. Precisely because Congressmen realized over the years that they lacked the information on which to criticize Executive policy or to suggest changes, they have opted out from an active role in the field of foreign policy. But by the same token, it was the executive branch itself which was denying them this information. So that what we saw was one more confirmation of the axiom on which I think our Constitution was originally built, which is, "power corrupts -- even Americans."