I see your point Manfred, but a lot depends on
what the impact is and, in particular
how it is delivered. If something small and sharp hits a lens at speed - like a stone at a race track, a filter might not do much, but for others it works. Also, I think it is not just the glass that is the protective element (excuse the pun). The metal ring
itself actually provides protection against an impact at an angle. In the case of the impact I quoted the metal ring was clearly deformed and flattened quite badly, but it did that instead of the seat for the front element of the lens. The lateral forces dissipated to the filter that then shattered, but those forces that were not are transmitted down the lens more in line with its structure and so were more likely to be absorbed. Of course, much depends on the degree of forces (I don't recommend hitting a lens with a hammer to test this out).