The preload process starts to work after clearing all the cache and it stops working after all caches are created.
If you want the preload feature to work constantly, you need to enable the “Restart After Completed” option.
Why do I need this feature?
When a new post is published, clearing the entire cache can cause a lot of CPU usage because the cache is generated for each page. If your site has a lot of visitors, cache is created for too many pages at the same time and it causes CPU usage. In this case, instead of clearing the entire cache, generating the entire cache in time ensures much less CPU usage.
If the option is enabled, the preload creates cache and overwrite existing cache file. Thus you don’t need to clear the cache.
Note: You have to set a cron job as you can see in this tutorial if you want to use this feature.