Why would there be a switch in a high-voltage power supply to turn off the power if its cabinet is opened?

The switch described in this question is known as a safety interlock switch. Its primary purpose is to automatically disconnect the main electrical power whenever the cabinet or enclosure is opened, protecting anyone working on the equipment from accidentally touching live, high-voltage components. High-voltage power supplies, especially those used in older tube-style amplifiers, can hold lethal amounts of electricity. Distractors mentioning RF radiation are incorrect because power supplies generate DC or low-frequency AC voltages, not high-frequency radio waves. The interlock is strictly a life-safety feature to prevent accidental electrocution.