Boxout: A piece of text written to accompany a larger text and printed in a separate area of the page. 
I wrote about the reason in my initial reply but removed it for the sake of brevity. I am not a designer or publisher, so I might have used the term incorrectly. 
In short, boxouts work better in print than on a web page because the flow logic is different. In a browser, the layout of a long-form article is generally top-to-bottom involving lots of scrolling, particularly on smaller and narrower mobile devices. Boxouts in print material have a place on the page or pages in such a way they don’t break the main text. E.g., if they are smaller than a page or a 2-page spread, they tend to occupy less room than the text body. When they are larger, they tend to be at the end of the chapter or section, or relegated to the endnotes in the case of charts and figures.
The problem with the top-to-bottom paradigm, especially when the text width is narrow, is the text becomes a very tall wall and unpleasant to read and scroll through; therefore, challenging to follow. For this reason, newspapers tend to layout the articles with narrow columns, in short sentences that are spaced out like paragraphs. Body text is also simple, packed and hyphenated. If there are lists, they aren’t longer than 1-2 short (incomplete) sentences. And so on.
When one includes boxouts, multiple levels of chapters, sections, subsections, lists and nested structures, it gets so overwhelming that the reader may give up or get an unintended condescending vibe. It is especially difficult for readers with disabilities. Same with images, if they are taller than the viewport (e.g., as a result of being in portrait orientation), their readability goes down.
The _boxout_s on this page don’t appear to be responsive to me. On a mobile device (Android, Firefox), I am unable to scroll or zoom to see the rest of the box.
If the article becomes unwieldy, it may be a good idea to include a table of contents for the sections themselves. But then, it might be a good idea to split the article or rewrite it. 