It's a misunderstanding that only the user writes to config. It is meant for persistent user configuration, possibly generated by the app - the type you might want to back up. You are not supposed to understand or be able to read every file in there, just be able to know the usage.

.local is rather new, so .config has been misused for what was meant to go there. Some applications are broken and stuff caches in there (Google stuffs shader caches in there it seems?), but minor misuses is one thing - .config and .local are conceptually very close for example - intentional disobedience another.

The spec is well adopted (I have hundreds of folders in ~/.config, only tens in ~), and there is no reason not to follow it - especially for cross-platform things where path logic must already exist. There are just some projects that decide to stubbornly go against community and user wishes and disallow improvements.