Layout grids are a useful tool for composition in any visual design discipline, in that they facilitate the task of organizing and accurately positioning elements both horizontally and vertically on a page. They also play a useful role in the positioning of elements in ways that make a design aesthetically more pleasing. Some of these grids, such as the phi grid, are covered in Chapter 23. In Web design, the effectiveness of the use of grids is no different. They help ensure consistency in the pages of a website, which makes pages easier to produce. Beyond visual grids that allow you to just sketch out design layouts on paper or screen, over the years, there have been various types of grid frameworks introduced into Web design, all of which allow the Web author to specify the number and size of columns and rows, and the gaps between them. It is on this general principle that the CSS grid layout builds.
Source: Sklar David (2016), HTML: A Gentle Introduction to the Web’s Most Popular Language, O’Reilly Media; 1st edition.