Why connect power supply ground with chassis ground through a capacitor?

[swen_s]

I'm wondering about the capacitor in the schematic (marked with red). Why is there a need for it?

It connects power supply ground (dangerous side) to the chassis ground (safe side). And why is there sometimes also a resistor connected across it (several mega ohms)?

I suppose the resistor discharges the capacitor after power off.

 

[emresel]

I think you should search for X and Y caps. It is called Y cap.
As i know it is used to suppress EMI to obey European CE regulations.

 

[FvM]

I suppose the resistor discharges the capacitor after power off.

Something like this. The capacitance is too small to store enough energy to produce an electrical shock. But there's a risk that the capacitor is charged above it's voltage rating (and possibly damaged) by static electricity. The resistor drains any static electricity.