How to use the on/off pin on a voltage regulator such as LM2574?
Enable/Shutdown pins on voltage regulators are used from externally controlled circuits. Unlike most hobbyist circuits where a single regulator powers everything, more complex circuits might need to control multiple regulators for different purposes.

Even in a single regulator circuit, an on-off pin could be used by a microcontroller on that regulator to reset itself.

And some regulators allow the On/Off pin for rapid toggling. Essentially allowing PWM control of the regulator, for example, for led dimming.