Why lifting tackles need third party certifications?

This can be required by state law or due to contractual terms & conditions between the government and contractor for a specific project or this can be required due contractual terms between clients and contractors.

The purpose of third party inspection for all lifting  tackles to ensure that all the lifting tackles are intact and sound safe for the operation. 

because they are the supplier of lifting accessories such as tackles. they should be inspected to ensure the safe or unsafe for using in the workplace with the frequency of time period. after the inspection, they should provide the certificate. 

Competent person should inspect the lifting like cranes and other equipment because He will observe ; Equipment shall be fit for its intended purpose and operating conditions and shall be designed to a recognized standard.The competent person also inspect that equipment shall be fitted with appropriate safety device.Internal Audit sometime they by pass the minor defect of equipment what could be a major mistake or occur an accident.In order to avoid unexpected accident Third party inspection is essential and effective .

If lifting equipment like cranes requires certification then same goes to lifting tackles, why? because they become part of the equipment during lift. Cranes are load tested by third party to check if their capacity are correct so it is wise to require that also to lifting tackles. Remember there are fake lifting tackles around.