
it didn't just beep when it was done, it would then nag, and beep every 30s until you opened the door.
The intent is to help you, not nag you.
If you've ever microwaved a frozen dinner or snack, you might have noticed that the instructions usually say something like "Microwave on high for three minutes then let stand in the microwave for two minutes."
Thus your microwave beeps repeatedly at a set interval after it is done cooking so you can track the post-cooking stand time without having to set another timer or guess.