If you have turned on the System Cache in Joomla's Global Configuration you will see that Joomla caches generated pages based on the user groups the currently logged in user is assigned to, as well as other data specific to each extension. This may cause a lot of cached data to be stored which are essentially used by only a few users or even a single user only. The expired cache is not deleted automatically which means that you are wasting disk space or memory, depending on your cache type. You need something to clean up the expired cache periodically. This task type can do that when you set its Clean-up type option to Expired Only.

Further to that, some sites may be updating their content with a scheduled task or with some other means which does not go through Joomla's web interface. While the Joomla interface will clean up the relevant cache section (e.g. com_content for Articles content) when you save or update content in its core components — and most third party components do the same — if you are skipping the Joomla interface this may not happen at all. This would mean that Joomla might be service the cached content. In some cases this can be a significant problem if you need to provide time-sensitive information, e.g. if you are listing availability of in-stock items being sold at a fire sale, currency exchange rates etc. In most cases it might not be practical disabling the system cache in Global Configuration for performance reasons but you'd still like to have Joomla's cache cleaned up periodically to avoid issues related to serving stale content. This task type can do that when you set its Clean-up type option to Delete All.