What is the benefit (rep?) if any to someone who votes to close a question? Is that benefit still there at all if the vote is wrong?
There isn't any benefit such as reputation or badges.
People vote to close questions without any sort of reward in order to keep the site clean and to remove questions that don't meet the requirements of the site - such as being off topic or too unclear to answer.
It's purely a volunteer activity, without any sort of reward or recognition for the effort of keeping the site running smoothly.