I can't find "official" XDG documentation of this, but the Arch Wiki describes, for instance, the XDG_DOCUMENTS_DIR and XDG_VIDEOS_DIR environment variables:

https://wiki.archlinux.org/index.php/XDG_user_directories

It looks like maybe the "official" documentation is here:

http://user-dirs.freedesktop.org/releases/xdg-user-dirs-0.17...

But I haven't gone through that carefully.

Why would it have to define those directories in particular?

You should be able to change the default settings for these directories by changing /etc/xdg/user-dirs.defaults (system-wide) and ~/.config/user-dirs.dirs (users override)