Answer: To clean the exterior and keypad of the microwave.
Step-by-step explanation:
It is proper to clean the exterior of the microwave including the keyboard. Although this isn't required to be done daily, Benjy saw his mum do it at the time she decided to clean it (maybe it was stained too) and being a child he picks up the act too.
It is advisable to clean not only the interior of your microwave but the exterior top including the keypad. Cleaning the keypad prevents dirt and dust from staying in them and causing the keypads to get stuck.
I hope this helps