It is redundant. And it is typically just noise.
But it can be used in a context where you want to make it very clear that you do not want the listeners/readers to think of multiple possibilities that might be judged to be of similar importance/magnitude/whatever.
When you want to say, in effect, "Sure, I know you might be thinking of more than one, and you might find it difficult to choose, but please pick one."
IOW, yes, it is used only for emphasis: emphasizing that a single choice must be made. There might be several close contenders, but only one wins.
And it might be that there is no single one. And that too is a reason this expression can be used. If there is a tie, then the answer is that there is no single winner.