The numeric keypad on a microwave is indeed functional, and you can press the numbers 7, 8, and 9. These numbers are commonly used to input specific cooking times or power levels. The misconception that these buttons have no function might stem from the fact that many people primarily use microwave ovens for basic tasks like heating or reheating food, which often require shorter cooking times in increments of seconds or minutes.
However, if you need to input longer cooking times, such as several minutes or hours, pressing the numbers 7, 8, or 9 becomes necessary. For example, if you want to cook something for 7 minutes and 30 seconds, you would press the numbers 7, 3, and 0 on the microwave keypad. Similarly, if you need to set a power level above 10, you can use these buttons. It's worth noting that the specific functionality of a microwave can vary depending on the model and manufacturer, but the presence of the numeric keypad is meant to provide flexibility in programming various cooking settings.