This is about use of earth screw terminals which are often provided in power supplies.
Below is an Embedded Switch Mode Power Supply with a metallic case:
Here I can understand how the earth wiring is useful in case of line is touching to the case. This would save someone being exposed to line voltage.
On the other hand below is a DIN Rail Power Supply with a plastic case:
It also has an earth terminal. In this case how is the earth wiring useful since the case is already insulator itself (plastic)?
Best Answer
EMC is the first thought - earthing certain line filters inside the plastic case may be a requirement for correctly using this power supply.
However, the earth symbol has a circle around it so it means "protective earth" (or PE) i.e. it is used for reasons of safety and the circled earth symbol is usually implying that the unit is built and CE marked so, it's mandatory to provide a PE wire.
So, that's its main function but, because it is always required to be connected to a PE, the PE may also be used for EMC reasons and the CE marking/EU directive applied might require it for these secondary reasons.
The unit appears to have CE mark so check its documentation to see what that says about the PE. As for what it protects, it's possible that the step-down transformer relies on a PE to protect the output circuits from rising to unsafe levels. Maybe the output is actually connected to PE?