The cable works. That is not the same as passing.

Plug a laptop into a new port, see a link light, get an address, load a page. It works. That test tells you almost nothing about whether the cabling meets the category it was sold as.

Copper links fail slowly and quietly. A pair untwisted a few centimetres too far at the punch down, a bend tighter than the radius allows, a run laid alongside power for twenty metres: none of these stop a gigabit link on a quiet afternoon. They show up as retransmissions under load, as a port that drops during the busiest hour, as an intermittent fault that three engineers cannot reproduce.

Certification catches all of it at installation. A calibrated tester measures insertion loss, return loss, crosstalk and length against the standard for the category, and reports pass or fail per link. There is no opinion in it.

What matters is receiving the results. A certification report per port, tied to the labelling scheme and the port schedule, is a baseline you can measure against in three years when something changes. It also settles the argument about whether a fault is in the cabling or in the equipment, which is otherwise a week of everyone blaming the other party.

The cost of certifying is small next to the cost of not certifying: chasing an intermittent fault through a live building, or re-terminating a floor because the installation was never proven. It is the cheapest insurance in the whole project.

If a supplier will not certify, that is the answer to the question you were asking.