How do closed and deleted questions affect our % Answered ratio? When should we vote to close?
Voting to close questions is a good thing.
When you see a question that has comments asking for clarification, and the original poster (OP) doesn't fix the question within a few days, vote to close it as "unclear what you're asking". OP will receive notification about the question being put on hold, which offers a final chance to fix the question. After a few days the status automatically moves to closed, and eventually is automatically deleted.
Closing vague or abandoned questions improves the quality and searchability of the site, as well as our % Answered ratio (as closed questions are removed from the pool).
This is an excellent overview of deciding when to vote for a question's closure.
And check out this link if you want to learn exactly when questions are automatically closed and deleted.