Shadows are one of the key features for creating realistic and believable virtual scenes. Whether hard or soft or physically correct, shadows are very crucial in providing important visual cues. Shadows can be an important aspect of gameplay also. For example, it plays an important role in CounterStrike by allowing us to see enemies around corners, coming through doors, etc.
Real-Time Shadows is an active area of research. There’s still no single fool-proof technique of rendering shadows that can handle all the cases. Well, there exists a way to render high quality physically correct shadows through ray-tracing but that’s not fast enough to render in real-time on current hardware. So a better way to say this is that there’s no real-time hack of rendering high quality, flicker/ jaggies free, physically correct, etc shadows yet.