Why do rock climbers use ropes?

In the good old days, hemp ropes were the main ones used, and rarely, cotton. The modern and superior alternatives are such materials as Nylon, terylene, and polypropylene. Terylene has similar properties to nylon, but it has a lower amount of elastic stretch - thus you are brought up more sharply after a fall. Nylon has a suitable degree of elastic stretch and most climbers prefer it. [Rubber would be even more gentle, and finds use in the Bungy Jump industry.] Polypropylene comes in a range of formulations and is a comfortable rope for outdoors use, but its serious defect for cold weather use is that it may have a glass transition temperature not far below 0 deg C. This is the temperature at which the material turns brittle. This is considered by climbers to not be a good thing. Nowadays, the kernmantle style of rope construction is by far the most favoured. This has a stranded core in the rope, protected by a close braided outer wear layer. [You'll often see this style of rope used in yachting, because of its superior non kink characteristics.] This style of construction helps with protecting the fall resisting properties of the rope from casual abrasion.