Win a contract of any size in Southwest Florida and somebody will ask for a certificate. Send the wrong one, or assume it does more than it does, and the job stalls at exactly the point where stalling is most expensive.
What it is
A certificate of insurance is a one-page summary saying which policies existed, with which limits, on the day it was issued. It is evidence. It is not the policy, it does not amend the policy, and it confers no rights on the person holding it. That last point is printed on the form itself and read by almost nobody.
Which means a certificate cannot make somebody an additional insured. Additional insured status comes from an endorsement to the policy, and the certificate merely reports that the endorsement exists. If a contract requires the status and you send a certificate without the endorsement behind it, you have satisfied the paperwork and not the requirement.
Reading one before you sign
Check that the named insured matches the entity signing the contract, which sounds obvious and fails constantly where an owner trades under more than one name. Check the limits against what the contract demands rather than what you assumed. Check whether the contract wants primary and non-contributory wording, or a waiver of subrogation, both of which are endorsements and both of which affect your premium.
Send us the insurance clause before you sign, not after. Reading it takes a few minutes and it is far cheaper than discovering on the first day of a job that the policy you hold cannot produce the certificate the contract requires.