Why do leather cables need to be connected to a tail fiber

A fiber pigtail is a single, short, usually, optical fiber that has an optical connector pre-installed on one end and a length of exposed fiber at the other end. The end of the pigtail is and to a single fiber of a multi-fiber trunk. Splicing of pigtails to each fiber in the trunk "breaks out" the multi-fiber cable into its component fibers for connection to the end equipment. [pdf]