Rich Site Summary is a small text file, usually named rss.xml or feed.xml, created to list a site's recent posts in a standardized format (title, link, date, and content).
It's not meant to be read directly by humans, but rather by a feed reader that periodically checks the file and shows you anything new. Lots of sites still publish one even though it's an old format, so it doesn't hurt to check if your favorite site has one.
I wanted a way for people to follow along without having to remember to check the site or sign up for a newsletter. Any feed reader can poll the feed without needing accounts, tracking, or middleman platforms.
Limiting the feed to the stuff/ directory is deliberate. I feel like that is the main focus of the site, while all the other pages are extra content or non-essential references rather than news.