Ropes are not actually a distinct item type - they are merely what chains are called when they are made of cloth. They are both listed under the 'chains' heading in the z-stocks menu, are stored in finished goods stockpiles, and are generally interchangeable.
Taking a rope with you at embark lets you construct a well more quickly. While usually it's much cheaper to take raw materials and produce ready goods after embark, item value of a rope is 10 vs. 6 of the thread. The dwarves will not enjoy using a base quality item as much, but cost-wise it matters 3 times less than the mechanism.