An EMS protects from high or low voltage (low voltage damages control systems),
High or low frequency (also damages electronics and control systems),
Miswired and missing grounds. If hot and cold (return) wires are mixed up, that can result in a “skin hot” electrical shock, a dangerous condition.
I had a plug in my garage that had no ground, EMS wouldn’t pass power.
We camped at an old campground for a second time at a different location during a packed weekend during a high heat index warning. In PA, 108 temps incredible humidity. Everyone’s AC full blast, voltages kept dipping down to 103 vac. My power shut down at 106.