The rectangle that encloses every frame of your film becomes the window through which the viewer looks into the world you have created. For the duration of the film, you want this to be their reality. However, this “window” is unlike reality in that said viewer does not have the ability to look around and take in more information than you give them; they are limited to only the knowledge you provide them through visual and aural means.
Think of the way you set up each shot as a tool that you can use to shape your viewers’ perception of every moment. If you’re going to provide them with this information, you want it to appear in a way that draws the eye to its most important parts so that these parts are quickly recognizable.