Repetition creates landmarks
When headings, images and captions align, a page is easier to scan. Readers can identify related sections more quickly. A grid defines these relationships without requiring every element to be the same width.
Space does work too
A gap between sections signals a change of topic; a small gap connects a caption to an image. Random spacing makes relationships unclear. Choose a few spacing values and use them consistently.
Break the rule with a purpose
A large quotation or illustration can extend beyond the usual column. This is more effective when the surrounding layout has order. Tie exceptions to meaningful emphasis instead of adding them everywhere for variety.