These rubber strings are used on the ends of 3/4" cows tails to keep the carabiner in place with no sliding. By keeping the carabiner in place, it remains easy to manipulate, when clipping in & out of equipment. It also helps to prevent carabiner rotation in gear elements. This can help to lessen the chance that a carabiner finds its way into a situation where it might be cross-loaded. Lastly, the string, by assuring the carabiner stays in place, helps to assure that the carabiner is loaded properly along the spine, where it is strongest, when the time comes.
These are also very effective when used on quick draws for sport climbing, for the same reasons, as it keeps the carabiners in place & ready to load correctly.