Stringing wire in any environment can quickly go wrong. Dropped conductors can wreak havoc if precautions are not taken. In an energized environment, the result of losing control or dropping conductors has a greatly magnified risk.
Guard structures are the first type of protection conventionally used to prevent contact with energized lines. Ideally, guard structures are positioned so that whether it’s the unexpected loss of stringing tension or something as major as a dropped conductor, the conductor being pulled will not make contact with the energized lines. There are other requirements, too, one being non-automatic setting of breakers for the lines being crossed if it is not possible to de-energize and ground them.