Main idea questions summarize the chapter. Not only is this an important skill, but the immediate feedback will help students that may not understand what just happened.
Some main idea questions summarize the chapter in great depth. This is particularly important for dense texts like Macbeth.
For younger readers, and less dense texts, the summaries and ideas are often simpler. Here is one from Stuart Little.
Sometimes we use these questions to highlight themes and symbolism. Here is one from 11 Birthdays.