I can only imagine someone asking this if they a) don’t use the terminal except if Stackexchange says they should and b) have yet to try and cleanup a system that’s acquired cruft over a few years. If you don’t care about it, then let me flip that around and ask why you care if people use XDG? The people who care about it are the people in the spaces that concern it.
Off the top of my head this matters because:
it’s less clutter, especially if you’re browsing your system from terminal
it’s a single, specified place for user specific configs, session cache, application assets, etc. Why wouldn’t such important foundational things required for running apps not be in a well defined specification? Why just dump it gracelessly in the user’s root folder outside of pure sloppy laziness?
it makes uninstalling apps easier
it makes maintenance easier
it makes installing on new machines easier
It’ll be in /home anyways and I heard BSD had some issues with something that could be XDG.