On this commercial PVC membrane, notice the clean side is downhill and dirty side is uphill. When water runs across the copper cable it actually picks up tiny particles that distribute across the roof to prevent mold and algae growth, also inhibiting dirt accumulation. But these cables are actually a series of lightning rods, so if lightning strikes the building, its purpose would be to absorb the electrical charge and direct it to a grounding source to minimize damage to the building. This is relative to shingle roofing because the granules are designed with a copper core, which allows shingles to have an algae resistant quality.