Updated: Jan 20, 2020
Most houses, buildings, bridges and other types of infrastructure can withstand plenty of water pooling on their roofs, but ash is an entirely different kettle of fish. Its density means that the pressure an inch of volcanic ash puts on a structure is often too much, and consequently, things begin to collapse.
Now would be a good time to re-view Pierce Brosnan's "Dante's Peak" to be reminded of how deadly a volcanic eruption can be - though one has to scale down the "cinematic exaggerations" of the movie - viewing it now provides one with a basic understanding of the destructive power of eruptions.
In 1978, the Mayon eruption in Albay was particularly violent, days on end were dark grey blurs. Using our car to buy food meant the bringing of gallons of water to clear our windshields when the wiper blades can no longer function properly due to the heaviness of ash on the windshield.
So, if your galvanized roof is quite old, it maybe sensible to sweep away the ash else the roof may collapse on you because it wasn't engineered to hold tons of stationary volcanic ash.