In my neighborhood, people start with So when they are drawing a conclusion. Like this:

So Mark is off buying beer and Bill is hooking up the trailer. There will be a delay. Let's say we meet at the boat ramp at 3 o'clock.
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So you're saying that he watered the workers' beer?
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I once knew a guy, a lawyer, who could get a laugh simply by bellowing "So!" in a cynical, courtroom fashion.