Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations provide comprehensive safety information (readOnlyHint=true, destructiveHint=false, idempotentHint=true, openWorldHint=true), so the agent knows this is a safe, repeatable read operation. The description adds minimal behavioral context by specifying it's a GET operation, but doesn't disclose important details like what format the members are returned in, whether there's pagination, or any authentication requirements beyond what annotations imply.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.