Why must an artificial fly land gently on the water?

In the context of fishing, an artificial fly must land gently on the water to effectively mimic the natural behavior of insects and avoid alerting the fish to the presence of an imitation.

Fish possess specialized sensory organs known as lateral-line organs located along their head and body. These organs are highly sensitive to minute, local water displacements, allowing fish to detect and localize the movement of other animals nearby before physical contact occurs. Because these receptors are designed to perceive subtle vibrations and pressure changes, a heavy or clumsy landing of an artificial fly creates unnatural water disturbances that can signal to the fish that the object is not a natural food source, such as a caddisfly or other insects commonly imitated in fly-tying. By landing the fly gently, an angler minimizes these detectable displacements, increasing the likelihood that the fish will perceive the lure as legitimate prey.12