Switch Mode Power Supply: a varistor between Earth and the secondary's ground

Hi,

I'm developing a 20-watt switch mode power supply. The PWM controller
is the Viper 27.
So far everything is going fine.

I noticed on a competitor's power supply the presence of a Disc
Varistor (56 Volt) between Earth and the ground of the secondary.

This is quite surprinsig for me.

Can anybody explain me the reason of that choice?

Thanks in advance,
Enrico