The purpose of a whistle on an oven or stove top burner is to indicate that the water inside a kettle or pot has reached boiling point. When the water starts boiling, it creates steam, which escapes through a small opening in the kettle or pot lid. As the steam passes through the whistle, it causes the whistle to vibrate rapidly, producing a high-pitched sound.
The whistle serves as an audible signal to the cook that the water has boiled, prompting them to turn off the heat and attend to the boiling liquid. This feature is especially useful when you're busy with other tasks in the kitchen and may not be able to closely monitor the boiling process. It helps prevent the water from boiling over or the kettle from running dry, which can be a safety hazard or result in wasted energy.
The use of whistles is common in stovetop kettles and some pressure cookers. They are a convenient and traditional way to alert cooks to the boiling status of the water or contents without needing to constantly watch the stove or timer.