Choreography
Learning where to place each element on a slide allows it to achieve its purpose in the best way.
Much like a dance choreographer or an orchestra conductor, a designer must co-ordinate many different elements to create a harmonious composition. Our brains are prediction machines and harmony is very appealing as it reduces the amount of time needed to process information.
Harmony is about relationships, how one object relates to another. Consider a paragraph of text and an image. Do they fight for attention, distracting our brains, or do they co-exist peacefully, allowing our brain to carry on about its business.
Visually, how you arrange the elements can have an enormous effect on how somebody else comprehends them. Without choreography, it becomes very hard to lead the eye, to know where to start or to identify the most important element on the slide.