Benefit? It breaks up the text and makes it easier to read.
The page I showed is an entire page of text with no breaks. Looking through various games I own, most do not have entire page of text running down it. Some do have paragraph indentation like yours, but also two column layout and most of the time the text is also broken up into sections with sub-headings or the page has illustrations so the text doesn't cover the entire page.
If I use Shadowrun 5E as an example, yes its using paragraph indentation and no spacing between paragraphs, but it's also using a two-column layout and a lot of pages break up the text further with callout boxes and sub-headings.
Please don't read the following with an angry tone. I am actually very thankful to you for giving this feedback and helping out the project so much. I am only arguing to collect my thoughts and receive feedback, not out of any kind of emotional response. It's just hard to argue with someone over text without sounding upset, which is a feeling I don't want to convey.
Many of these handbooks do have an entire text of page running down them though, especially at the beginnings. Like page 4 of the D&D 5e PHB, Page 12 of the CoC Investigator's Handbook, Page 16 of V:tR, Page 19 of the Shadowrun 5e Quickstart, Page 11 of the Pathfinder 2e Core Rulebook, and Page 29 of the Star Wars REUP document. The only times any of those documents had line spacing is when separating sections, which this document already does. And this guidebook does include images on close to half of the pages (45% is close enough), which is actually a higher ratio of pages with images v total pages than most of the systems I have observed.
Keep in mind that these rulebooks also have significantly hellishly smaller font sizes than the standard 12 I am using for Domains of Shadow. The introduction section is of similar length to a lot of other system introductions and prefaces, but in a text size that is more accessible to those of us with imperfect eyesight. In fact, the introduction is actually significantly shorter than some of them (in terms of words), which if looking through the pages I recommended actually shows many examples of that (D&D's is a lot more rambling then I thought it would be). And almost none of these system's starting paragraphs I gave examples of are broken up into headings either, and there really isn't a reason I can think of as to why they would intentionally depart from traditional writing to do so. If this introduction is unreadable, than the introduction of almost every other major TTRPG would be just as unreadable.
The way I structured the introduction is very similar to the way that other system's make their's, they just do it in a smaller text size. I am worried departing from the standard here could very well make it harder to read then if I structured the intro in a way that no one has seen before.
That being said, this double column feature I am seeing in all of these rule-books is interesting, and something that slipped past me. I will need to do some research on why they do that and look into how I can incorporate it. Might have to start using Microsoft Word rather than Google Docs to pull of something like that though?