On the most basic level the lighting of a scene should be used for staging purposes; for example on a theatre set a spotlifht is aimed at the actors that are delivering dialogue to isolate them from their surroundings, so that the audience’s attention is focused on them rather than on anything else. In this context the audience suspends their disbelief and won’t question the incongruous light, because all attention is wrapped around the unfolding story. You might think that in less-staged settings such as feature film, the spotlight device is not feasible, whereas in fact it is widely used, it’s just done done more subtly.