Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already cover readOnlyHint and idempotentHint, so the safety profile is established. The description adds useful behavioral context by specifying that only published compounds are returned and by listing the returned fields, which is beyond what the annotations alone convey.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.