The certificate of insurance is important because your company can be held liable if you work with a subcontractor who causes property damage or other damage when working on behalf of your company. Even when you have a contract with the subcontractor spelling out the terms of the contractor, and that contract states that coverage is mandatory, if the coverage actually doesn't exist, then it does your business no good; you're still liable for damage. Proof of coverage from subcontractors is therefore vital.