Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Even though the schema covers the secret parameter well, the description adds actionable meaning: the secret must be the one sign_up returned, the data-key header cannot be used, and omitting a string yields a specific 401. This is more than the schema alone provides.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.