The use of strong lights and bright sunlight is almost inevitably going to cause some dark shadows. Strong, hard light creates harsh shadows and sharply defined shadow lines. These are unflattering, creating angular face lines and harsh areas of the scene. It is natural to want to relieve this tension in the composition. The use of fill-in Flash, or fill light is to help fill out those dark places and harsh shadows. More about Fill-in Flash; Fill-in Light; Fill; Fill Light in the “Photographic Glossary”.