I flagged a question to close it, and my flag has been declined. The question has been closed the same.
How can my flag not be useful, if the question has been closed?
The flags for closing a question, together other flags, are visible in a page to 10k users and moderators. The flag is automatically handled when the question is closed, or it is manually handled by a moderator.
It could also happen the question is voted to be closed from 5 different users, after the flag has been declined from a moderator. In that case, it just means the community have an opinion that is different from the opinion the moderator who handled the flag has.