This wonderful, short expression is so useful! It can be used to bring an unprofitable discussion to a swift conclusion. It can be used to stimulate thought in another. It can be used as a nasty retort—or as the response to one.
But, sweeping aside those uses, let me suggest one that might be profitable to preachers. If along the way, as he is preparing his message, a preacher keeps asking himself the question, it might become the most useful tool in his toolbox for producing sermons that have depth and relevance to his readers.
Pete, the preacher, has isolated three points [why not four, two, or even six sometimes?] that he wishes to make (one hopes that they were in the text, not forced upon it). He states the first point: Paul says that God commands us not to worry. To begin, it would be wise to convert “us” into “you,” thus making the sermon more personal. It might also pack even more authority if you drop the words “Paul says that.”
But, now, Pete asks himself our question—“So what?” So what, indeed! So, we ought to recognize the command is from God and, therefore, cannot be lightly brushed aside. Ask again: “So what? So, we ought to realize that a command disobeyed is sin. Once more: So what? So, we ought to recognize that God never commands His people to do anything that He will not empower them to obey. So what? So, impossible though it may seem, a believer can stop worrying. So what? So, stop it!
From there, he goes on to work out the other aspects of Philippians 4 that explain how this is possible, queerying each statement he makes with the question, So what? If he does so, and thoroughly answers it each time, he will have a wealth of sub-points from which to choose (providing depth), and what he says, more than likely, will be relevant. I say “more than likely” because he can mess up an otherwise good sermon in may ways.
“I’m not sure that I can do this well,” someone objects. My reply: “So what?” Meaning, “If you find this process helpful, but can’t do it, then learn to!”