This is indeed a subtle feature.
This aspect of the question bank was originally created by Jamie Pratt, in order that he could build the Question creation activity, so that is one use. Stopping questions created by students getting mixed up with the teacher's questions. (Although, it teacher wants to steal reuse the Students' questions, they can move them to a different area of the question bank.)
We also use this facility at the OU. For formally assessed quizzes, we control their creation and administration using our Workflow block. As part of that, we put all the questions in the quiz area, and then we use Permission overrides, so that after the questions have been approved by a reviewer, they cannot be edited any more. This also reduced the chance that summative questions are accidentally included in a practice quiz.