One reason to use liquid fill is that liquid fill lubricates the movement's moving parts which reduces the effects of wear and extends the lifespan of the gauge.
Is extended gauge life worth the price of quality liquid fill fluid?
Pulsating pressures are often fluctuations of the pressure with large amplitude oscillations of the pointer. Such oscillations cannot be dampened by the use of filling liquid only. A snubber is the best solution for pulsating pressures.
A filling liquid will only be usefull for mild vibrations which give rise to small amplitude oscillations of the pointer.
As already said by others, the filling liquid has the advantage of lubricating the gearing mechanisme but besides that it also keeps the moisture out of the case, preventing the pressure gauge from corrosion.
>Is it necessary to use liquid filled pressure gauges, since
>snubber is used on discharge of pumps and compressors?
Yes, I wouldn't consider putting a pressure gauge on the discharge of a centrifugal pump without liquid fill, if you fit a dry gauge it will rattle to bits very quickly not so much from pressure fluctuations as just mechanical vibration, Glycerine or Silicon oil would be my choice. I don't believe a snubber will protect the gauge against mechanical vibration on fluid service.
As a rule I don't use fill fluid for gas service because the medium is compressible here is where a snubber might be useful.