A waterproof sail wchich is non not taut correctly can flatter dangerously in windy conditions, transmitting shocks and vibrations to the anchors.
The waterproof sail is likely to tear and the anchors are exposed to even a lot of stress strong that can damage them.
The tighter a sail is, the more it can "cut the wind ". The fabric stays still, it does not vibrate and does not transmit the force of the wind to the anchors.
This also applies to the breathable shade sail , the more taut and the more its perimeter will cut the wind more effectively!