This comment from a systemd develop is what is (the most important thing) wrong with systemd. Its not the first time I have seen comments like this and its not hard to find them.
> Reading the comments by systemd developers on that bug, I am now confused about what the --purge option is actually for.
Its intended for package management (basically the entire sd-tmpfiles is intended primarily for package management), specifically when uninstalling a package it is used to clean up the dirs/files created by that package (while on the other end it is used to create dirs/files when installing).
They don't parse the tmpfiles.d configs nor are they the ones that create the directories. sd-tmpfiles effectivly is part of the package manager (if they make use of it during uninstall) just like sd-sysusers for creating service users. They indirectly use it by installing files to tmpfiles.d, so it does somewhat follow that they should use it to remove the affected files again.
IIRC idealy services wouldn't even need any tmpfiles.d entries at all and would rely on the service StateDirectory= and RuntimeDirectory= but there are also a decent amount of other cases (mostly programs that require config files in /etc/ which idealy shouldn't be required) where there need to be some other dirs/files present.
Found via The Register: https://www.theregister.com/2024/06/20/systemd_2561_data_wip...
Reading the comments by systemd developers on that bug, I am now confused about what the --purge option is actually for.