That is an interesting question. We have all kinds of sayings that we pass between our lips without even thinking about them. It is good and instructive to think about them sometime. "For the sake of God," or "for Heaven's sake," and so forth are even uttered by self-avowed atheists, who will assure you most strenously that they DO NOT believe in God.
Speaking as an American and assuming (perhaps wrongly) that most of us on HubPages are Americans, one can say that as Westerners, we share a common Judeo-Christian heritage. Or perhaps its more accurate to say we all share some understanding of the Judeo-Christian tradition as part of our collective cultural heritage.
What is this invocation of "God," which, again, you will even hear from self-avowed atheists? Well, "God" is understood to be the ultimate authority, is He/She/It not? By attaching your statement to "God," one is perhaps saying that something is unalterably obvious.
But one might say something like: "Hurry up, for the sake of God?" Perhaps we said, even by a self-avowed atheist, this invocation is an unconscious appeal to some outside force, if necessary, to quicken you (the person to whom the plea "Hurry up" is being made). Just some thoughts.
Thanks for the question.
Take it easy.