What is the function of the X-ray machine exposure handbrake in the X-ray machine system?

Those who are familiar with the X-ray machine system should know that the exposure time of the X-ray machine’s filming system needs to be set in the system in advance, so what is the role of the handbrake as its control switch? Why do you need a physical handbrake to control exposure instead of direct computer control?
The hand brake of the X-ray machine controls the opening and closing of the power supply, but the exposure time of the X-ray machine itself needs to be set in advance in the system. On the one hand, it is because of the stability and convenience of its exposure handbrake. If it is controlled by a computer, the stability of the electronic control system is far less than that of physical control. On the one hand, for the operator, the exposure handbrake is also more convenient to use.