Or it is so the Nazgul and Sauron can waste time in debate for several days trying to work out Why Boulders. Just to mess with them, he threw in something random.
Brilliant! Now I’m seeing Elrond and Gandalf discussing strategy a few days before...
Elrond: “...and Glorfindel will cover the road. That takes care of the searchers. Now for the defense of the ford, I’ve got a company of my finest archers with incendiary arrows waiting in ambush. They’ll be perfectly positioned to strike any enemies approaching the ford. Even if they can’t destroy the Nazgûl, the flaming projectiles should deter them, and will certainly kill their horses, enabling Frodo’s safe passage.”
Gandalf: “No no, that won’t do at all. We should unleash a huge flood.”
E: “A flood? In addition to he archers?”
G: “No, there won’t be any archers. Just a huge flood. As big as you can make it.”
E: “Okaaaay... well I suppose that would wash away the Nazgûl, but-”
G: “And I’ll add spiritual cavalry on top of the water, and spiritual boulders too.”
E: “Spiritual boulders? What will all that do?”
G: “Do? Well, nothing really, but the cavalry will look really cool, and the boulders will make an awesome grinding noise.”
E: “Grinding noise? I fail to comprehend what-”
G: “It’ll be amazing. Trust me.”
E: “This sounds like overkill. What if the flood engulfs Frodo along with the Nazgûl? My archers could shoot the enemy with great precision and-”
G: “Nope. Has to be a flood. Might engulf Frodo, yes, but you have to take these risks if you want to be in the history books, Elrond.”
E: “Ugh, fine, we’ll do the flood. This had better work, Gandalf.”
G: “Everything will be fine, you’ll see. Now the only thing that could make this plan better is some spiritual sea serpents, which I might-”
E: “Don’t push it.”