It basically allows you to define which paths are used for the Downloads, Documents, Videos, etc.. types of directories.

Not the commenter you replied to, but I change my XDG directory names to be lowercase and start with different letters. For example, Desktop, becomes "drop" (as in pick it up and put it somewhere else) and Downloads is a subdirectory dl. A program that would otherwise save to "Downloads" now saves to "~/drop/dl". When I setup my machines I run a script including the line xdg-user-dirs-update --set DESKTOP "drop" to update the XDG directory and I delete "Desketop". So og commenter has the option of updating their userdirs to be nested in their username if they wanted to avoid symlinking. Here's the relevant arch wiki page and xdg freedesktop page.

Reading back, my comment sounds snarky, but I was genuinely trying to be helpful.

Like what pemptago was describing, instead of symlinking your directories to /home/username/username, you could simply update that file and achieve the same effect, but in a more “official” way that may prove more robust.