when is this setting used?
Logs, to name one. Without it, the time in the container will be UTC without it.
To add to @Roxedus answer:
code-server saving files with a time,
MariaDB data alterations,
Domoticz with script running,
Syncthing file/sync times, …
Some containers might not use it for the application, but still for logs and being in the container template.