What does "Show correct answer" do?

When the Show Correct Answer checkbox is enabled, it provides two benefits: 

The first is Trainees who have no more attempts left on the question, or if they answered incorrectly, will be presented with an "Answers" button. Clicking the "Answers" button will reveal what the correct answer was. 

If the question were answered correctly, the explanation would automatically be displayed, and the Answers button would not be displayed.

Trainee View:

2. The second benefit is that Trainers are presented with an "Explanation of correct answer" box for each question. This box allows the Trainer to explain the reasoning behind the correct answer. This explanation will show up either after the trainee selects the correct answer or after they have run out of attempts, it will show up when they click the "Answer" button.  This will not show up in the course preview for Trainers.

Administrator/Trainer view:

By default, "Show Correct Answer" is enabled when creating a quiz question. You can add the details below the "Correct Answer Explanation" once a question type has been selected.