Not every Certificate of Analysis was produced the same way. Some are generated by the manufacturer's own in-house lab as part of routine production QC; others are produced by an independent third-party laboratory with no stake in the result. Both have a role, but they answer slightly different questions.
What in-house testing is good for
In-house QC is typically fast and run on every batch as a matter of course, which makes it well suited to catching gross manufacturing problems early — a batch that's badly out of spec gets flagged before it ever reaches a customer.
The limitation isn't the science, it's the independence: a manufacturer testing its own product has an obvious incentive, even if no impropriety is intended, and there's no external check on the result.
What independence adds
A third-party lab has no commercial stake in a batch passing or failing, and typically has no visibility into who supplied the sample or what result is expected. That structural separation is what gives an independent CoA more evidentiary weight than an in-house report alone.
It's also why serious suppliers tend to publish which independent lab performed the testing, rather than presenting results anonymously — a named, accredited lab is a claim that can actually be checked.
Reading the two together
The strongest quality picture combines both: in-house QC catching problems quickly at the point of manufacture, and periodic or per-batch independent testing providing an unbiased outside check on the same material. If a supplier can only offer one, an independent CoA is the more useful of the two to ask for.