E.g. for quick provisioning of containers or virtual machines, this is also to make sure the required directories always exist. In a normal distribution, /home already exists, so systemd-tmpfiles does nothing, but there are cases where you want to setup a standard directory structure and this is a declarative alternative to scripts with a lot of mkdir, chmod and chown.
The name systemd-tmpfiles is kind of historic at this point, but wasn’t changed due to backwards compatibility and all.