Metal roofs with smooth surfaces (native metal like copper or zinc, or painted metal) are generally quite slippery when wet. In winter, under the right conditions, this allows accumulated snow (and ice) to avalanche off roofs. If the fall area has no traffic or things that can get damaged, that’s probably a good thing, as there is less weight on the roof. If the snow ends up hitting people (your kids, your postman/woman, your visiting friends), or sensitive property (your car, your expensive BBQ, your heat pump, your eavestroughs/gutters), then that’s not a good thing. Unfortunately, if the snow control methods were not part of the original roof design and installation, then intervening after the roof is installed is often an area ripe with unintended consequences.