What is the purpose of a safety wire through a turnbuckle used to tension guy lines?
Why A turnbuckle is just a threaded barrel with a right-hand thread on one end and a left-hand thread on the other, so wind buffeting and vibration can slowly spin the barrel and let the guy line go slack. Running a safety wire (or cotter pin) through the barrel and around the eye bolts locks the barrel so it cannot rotate, keeping the guy tension you set. This is standard hardware practice on towers, where a slack guy can let the structure sway or collapse.
Watch out The idea that the wire catches the guy if the turnbuckle breaks sounds plausible, but a thin safety wire could never hold the tension of a guy line; its only job is to stop rotation. Guy grounding and tension measurement are handled by separate hardware such as ground clamps and tension gauges.
Safety wire = anti-unscrew. Think of it as a lock nut for the turnbuckle, not a backup rope.