Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
The description adds behavioral context beyond annotations: the mailbox is temporary by default, the domain falls back to the tenant default, and the owner is always tied to the API key. Since annotations already declare readOnlyHint=false, idempotentHint=false, and destructiveHint=false, this supplementary context is valuable and non-redundant.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.