The fickleness of fortune, the temporary nature of its generosity and interest in a man is what the monk wants to warn the others. And in his tale, the Monk reveals that one trait of Fortune is that she whimsically supports and then changes her mind about the person she favors without no warning.
By putting it as a matter-of-fact statement (assuming everything else was action packed) there is a massive shift that creates contrast in the piece and leaves a very concise point to shock/leave the reader with.
