Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly, idempotent, non-destructive and closed-world behavior, so the safety profile is covered. The description adds value beyond that by disclosing what the result contains (observed time and expiry) and explicitly ruling out side effects ('does not reserve capacity or initiate work'), which is exactly the kind of context annotations cannot express.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.