There is a version of this conversation that focuses on brand and trust. Bad documentation makes your product look unprofessional. Users lose confidence. That is true, and it matters.
But the more direct cost is time.
Every support ticket that exists because documentation was unclear represents a cost. Someone on your team answers it. Someone at the other end sent it. Both of them spent time that the documentation should have saved.
The second cost is review cycles. Documentation that is not grounded in source material goes back for revision. Sometimes once. Sometimes three times. Teams absorb this as normal. It is not normal. It is a process failure that looks like a writing failure.
The third cost is adoption. Features that are poorly documented get used less. The feature existed. The value was real. The documentation was the gap between the feature and the outcome.