CRAP tells you to put related things close together (proximity) and to make like things look alike (repetition), but it does not explain why those moves work. The Gestalt principles, which come from early-twentieth-century perception research, do. They describe how the human eye automatically organizes what it sees into groups and wholes, before conscious thought. A handful matter most for document design: Proximity: elements placed near one another are read as related. (This is the same insight CRAP borrows.) Similarity: elements that share a color, shape, size, or font are read as belonging together, even when they sit far apart. Continuity: the eye follows lines and alignments and expects them to continue, which is part of why alignment feels orderly. Closure: the eye fills in gaps to perceive complete shapes, which is how a logo made of fragments still reads as a whole. Figure and ground: we separate a foreground object from its background, which is the work that white space and contrast quietly do. Common region: elements inside a shared boundary, such as a box or a shaded panel, are read as a group even without proximity.
This is a great checklist to use when proofread, peer review, and just review any type of work. It makes things easier to read as well as scan.