Game worlds can be a fully interactive environment where realism usually plays an important role and actions occur coherently to the player's eye. To achieve these sensations, you will need to query the physics world. A good example is Line of Sight (LOS) tests for artificial intelligence or even a 3D sound system, which should attenuate the sound received from the vicinity if there is any wall between the audio source and the listener. This check is easy to carry out by casting a ray between the two world points.

Another way of receiving direct feedback from the physics world is area queries, which can help you to detect dynamic obstacles and jump to avoid them. Even less common cases are where lots of people are upon a wooden bridge, so you can calculate their total weight and make them fall if it reaches a certain threshold.

Useful, right?