Tim,
If you still don't understand the principle, read Peter Diedrichs's post (https://moodle.org/mod/forum/discuss.php?d=432354 ↗). Peter clearly explains that the questions of a quiz can be all over the place in the question bank and that it is laborious to trace them, for example for archiving reasons. A good user experience would be to have a command, in the quiz Questions page, to export questions from the quiz.
If you still don't understand the principle, read Peter Diedrichs's post (https://moodle.org/mod/forum/discuss.php?d=432354 ↗). Peter clearly explains that the questions of a quiz can be all over the place in the question bank and that it is laborious to trace them, for example for archiving reasons. A good user experience would be to have a command, in the quiz Questions page, to export questions from the quiz.