In Switzerland, there are signs of the same size positioned at the beginning and end of bridges, overpasses, etc... along motorways and some secondary roads.
Likewise, there is no reference to them in driving education material, and they are different in each canton. In Valais and Bern cantons, they show the picture of a truck with a giant shovel or blade.
The commonality with Austria is the higher exposure to snowfalls, due to the geographic situation.
My guess about their usefulness would be: when the roads have to be cleared of snow in winter, the snowplow operators know when they are going to enter an overpass section. They may take extra caution not to push snow off the side that may fall on the other road below, and resume normal procedure after they leave the overpass section, marked by the second sign.