Noah, like Meathman says its to protect you against claims, Its a very expensive insurance and covers a lot of things and all for your and his protection, like if a slate blew off in the wind and hit a person or a car during construction or your postman got injured or fell over a concrete block in your driveway or as above someone he had working for him fell off a ladder, you will have the satisfaction of knowing that it won't be you they would be claiming off but off builders insurance,