The answer is simple: a mooring line serves to moor your ship on a dock, on the quayside or on a bollard. Sometimes, waves, caused by other boats or the wind, cause the moored ship to rock back and forth, requiring a certain amount of compensation. If the mooring line doesn't have a certain degree of elongation, the kinetic energy of the wave puts unhindered force on the cleats or grommets on which the rope is fastened to, causing possible damage. A rope with sufficient elongation absorbs these stresses and "defuses" the wave's kinetic energy, thus protecting your equipment and boat from damage.
Another important feature of the mooring line is its breaking force. As a rule of thumb, a mooring line must be able to secure several tons of weight over several years, in all types of weather conditions. Therefore, they should have a high breaking force and not harden or become porous and brittle, even in the case of fluctuating weathering conditions. Gleistein mooring lines fulfill all of these requirements and also have flexible sheathings for easy handling. On top of this, they are incredibly durable and absord a very low amount of water.
Mooring lines should have a certain degree of elongation and ideally should not sink immediately once they fall into water. But why should a mooring line have a certain amount of elongation?