Wouldn't coarse sandy paint also serve to slow down water in a downpour? But I guess they have been there 400 odd years, so I suppose this has some advantage.
The flat parts of each step slow the waters downward momentum much better than a slope would.
Sounds like that coarse sand would also leech into the water.
The are already surfaces that are painted and have sand mixed in to make surfaces less slippery. The bumps and pits would slow down water as it sought its level toward the gutters.