It is one of the very basics in desktop publishing: the grid. Such a grid provides some sort of frame or scaffolding which the entire layout of a publication, including copy and pictures, is based on. Grids can be set up differently, but it should always have a good balance of "rigid structure" and "flexible design". To give you an example: For my architecture magazine I chose a 12 column grid, so I can easily place the copy in 2 or 3 columns, but also have enough white space around nicely designed quotes and pictures. Everything has enough space to "breathe".
No matter how a grid is set up: It is very important for the editorial designer to stick to it. But why? Why is a grid of such importance?
The grid contributes to a harmonious, visual unity of all stories/chapters. A systematic arrangement of copy and pictures based on a grid, allows the reader to easily navigate through the publication without any hassle. This also accounts for the chapter and page number. Imagine, chapter and page number would be somewhere else on every page, or they are displaced by just a few millimeters on every page. The reader would have massive difficulties to orientate himself, the magazine/book would all of a sudden be "difficult to read". It is important that flicking through is conceived logical and without hurdles.