It first justifies why Florian Triangle was considered a cursed place even before Moria arrived with Thriller Bark. Secondly, it plays with naval legends and the mythical figure of the Umibozu; the shadows whole purpose is to emphasize that oceans are mysterious places, and therefore explaining them would take away what made them interesting in the first place.
If you explained them as giant natural pillars you'd be instantly taking the mystery from them, and therefore what made them interesting in the first place. Giant natural pillars have no romance behind them; unknown shadows do.