Why companies need third party certification for the equipment?

Basically it all started as mistrust of one organization to another.
Since maintaining equipment in proper working condition is costly, some companies tried to take shortcuts and failed to ensure quality of their product and and services.
Now the company contracting another one, wanted an assurance that the contractor will be able to deliver the business in timely and efficient manner, of required quality.
So since they couldn’t trust the promises of the contractor they required the contractor to hire a third party consultancy to inspect an issue certificates.
Hope this clarifies the issue.