Shock absorption and stretch combine to reduce fore-aft motion when spring lines wont work.
Securing a small boat between pilings in a wrong-sized slip is a common challenge. The dock line angles from the dolphins (outlying pilings) are too narrow for a beam wind, allowing the boat to dance around, increasing forces, chafe, and even making it difficult to stand in the cockpit. During a recent winter near-gale we measured dockline forces on several smaller boats that reached four times higher than the static wind load. If the recommended size dockline was used, the rope would be operating beyond its working load limit in real storms and could fail. Increasing the line diameter would result in more jerking and chafe.