As humans we love the majesty of nature. Who can fail to be impressed by such majestic cataracts as the Niagara falls or the Victoria falls? Someone said that falling is fine, it is hitting the ground that kills you. Same thing with the water in the waterfall. When it hits the end, enormous chaos is generated both visibly as spume and invisibly in the water.
We do not usually try riding a waterfall in a barrel. We can harness it for electric power. But if we do, we certainly do not want to ride it in a barrel.
Agile is also a Waterfall
Agile software development methodologies are about breaking the waterfall into safe, manageable steps. The impact is not saved for the end in a big bang delivery. Instead it is taken one step at a time. The water does not flow as chaotically. It is more like a laminar flow.