Practically almost all the typographic rules that are followed today are not a recent invention, most of them come from the first reunification of criteria regarding the book design and composition back in the 18th century.
These rules are innumerable, among them the indentation for the paragraph break. On pages with several text columns, the graphical way of showing that a paragraph ended giving way to the next is a blank space regularly defined by m-quads = a block the same width and height as the font size and without print relief. Once printed this empty space also serves to make the reading more relaxing.
It was also a way for the typesetter to follow a certain rhythm while assembling the page in a metallic type form, where he/she not only had to place letter by letter manually, but also had to do it the other way around. All typographic blanks are guidelines to better understand and follow the page composition.
Image source Wikimedia.org
In web pages it is rare to find these spaces and other typographic rules applicable to the printed edition because it is unlikely:
- Find endless texts like in a book
- Have long paragraphs arranged in two or more columns
On the other hand, a book or page has physical limits, the page itself or the amount that should be in a book, while a website has no those limits. On many web pages, paragraph breaks don't need indentation because they usually use another resources like adding an empty line as a space between paragraphs: