What is it???
 

on Jibs and cranes, Sometimes I see a cable that runs along the top of the structure and it is arched(or humped somewhere in the middle) and connected at both ends of the crane itself. Is this for structural support because the crane is so long....or does this serve some other function that I am not seeing. Thanks for the help.

Structural support.

Some jibs that can be built in different lengths only need it past a certain length, so sometimes you'll see the model with and sometimes without.