Question Banks exist as a place to house quiz questions for repeated uses. 

One of the most common use cases would be you have 50 total questions in a question bank and you want 20 random questions to be pulled from the QB and given to the user. 

In a similar vain, you can create sub-question banks to house seperate topics (ie: you have 10 questions about topic A, 10 questions about topic B and 10 questions about topic C and then a quiz is created the pulls 3 questions from each topic to make sure users are assessed on the desired content).