Solar panels work by converting light into electricity. If there is dirt on the surface of the solar panel, it can reduce how much light is absorbed by that area, reducing efficiency. Cleaning solar panels reduce dust, debris, and other contaminants such as leaves and bugs, which could further reduce efficiency. If you keep up with regular maintenance, you will also eliminate any buildup of dirt or dust on the surface of your solar panels that would otherwise continue to reduce their efficiency over time.