Purpose of high pass filter for AC Mains?
Is it necessary?


If it isn't there you have full mains voltage on the bridge rectifier.

The capacitor has an impedance to AC given by \$ Z = \frac {1}{\omega C} = \frac {1}{2 \pi f C} \$ where f is the mains frequency. The result is that it will drop the voltage to the load and, since it is a capacitor, there will be very little power dissipated in it and this eliminates the heating problem associated with a resistor voltage dropper.

Please be aware that this circuit provides no isolation from the mains and that the low voltage side must be considered live.