Why check before launch rather than after?
Because paid traffic is the most expensive visitor you will ever buy, and every visitor sent to an unready page is spent. After launch, the only evidence is a disappointing conversion rate, which rarely says why.
What is the most common failure?
The broken promise. The ads say fastest, simplest, most trusted; the page never says any of it. On one live account, every campaign made promises its landing page never stated.
What can a check not tell you?
Whether the copy persuades. A readiness check judges what can be verified: structure, how clear the offer is, friction, proof, measurement. Whether it persuades is still a human call.