Why must the restraint chains (Halteketten) of a PTO drive shaft guard always be fastened to fixed points on the tractor and the implement?

1. To stop the guard sliding off the shaft as it telescopes in and out
2. To prevent the plastic guard from rotating together with the steel PTO shaft turning inside it
3. To carry the weight of the shaft when the implement is detached
4. To earth the implement against static electricity