over at grognardia the value of not having all the answers came up in this post: The Unknowable.
While I myself do enjoy long richly detailed histories and long hidden secrets driving the action behind the scenes RPG campaigns really don't need all that all the time.
By Example my current Mutant Future campaign takes place in the remains of a massive coastal metropolis that might have once looked like a 21st century American city. I have no idea what city it is, I have no idea how long ago "the apocalypse" happened, who killed who, why they did so or what weapons were involved. It will never matter to the campaign, even if we get a year or two of play out of it I can't imagine how it will ever be relevant. Time spent answering those questions would be wasted and only lead to more questions with answers that don't matter one bit or that will restrict me as GM.
My campaign has plenty of mysteries to resolve in just the remains of one moribund megalopolis, I have no reason to worry about the whole world and the why behind it all is easy...to play a game and have fun.