The all-too-common phrase "God willing" or "Lord willing" ("insha'Allah" in Arabic, or "Lord willing and the creek don't rise" in Redneck) is one of those sayings whose purpose is still somewhat opaque to me. Although its usage indicates a meaning akin to "barring unforeseen circumstances," its heavy use among theists--particularly devout ones--seems like a verbal tic. Those afflicted by it have a sort of theistic Tourette's, and seemingly can't resist name-dropping their deity into every conversation--regardless of appropriateness.
After all, positing that a deity may decide to thwart one's plans adds nothing to the original statement that a simple expression of uncertainty couldn't cover. I find it interesting, however, that their omniscient and omnipotent god is used as a proactive excuse for the possibility that their own knowledge and/or abilities may be inadequate--shouldn't they be blaming a demonic influence instead?