Why climbing ropes should be selected carefully?

Dynamic and Static are two types of ropes generally used for mountaineering and sport climbing. 

Dynamic Rope (Generally between 8-10mm diameter)

While climbing there are chances of fall so the rope should have some elasticity to absorb the shock from sudden fall, thus a dynamic rope with approx. 10-12%  static elongation for maximum length of 5 mtrs is recommended for top climbing. It must be noted that the longer is the length of rope between the climber and the top anchor the greater would be its elongation and max 40% dynamic elongation is recommended to maintain the balance otherwise there are chances of the climber turning upside down.

Static Rope (Generally between 9-11mm diameter)

Whereas, while rappelling or descending the weight of the body hangs on the rope and if there is too much elasticity in the rope then it becomes difficult to balance the body weight on the slope. It is thus recommended to use static i.e. low stretch rope with max 1-5% of elasticity.

What are dry ropes?

During mountaineering the rope generally absorbs water from the snow and ice and not just gets heavier to use or carry but when frozen gets stiff as well. Thus, waterproof ropes are more reliable during climbing in the snow areas and monsoon seasons. 

A rope can have a dry core (inner threads) or dry sheath (outer covering) or both which can be considered as the most reliable dry rope with very less capacity to absorb moisture and thus easy to use.