Purge Cache

Hi,
the “Purge Cache” function is foreseen in the plugin settings.
The same function of the plugin is present in the WordPress top bar.
I didn’t understand how often I have to manually perform this operation.
Isn’t there a programmable purge cache option, such as every 24 hours?
Last question.
After clearing the cache I made a backup and the cache was once again full.
Do I need to empty it again after the site backup runs?

Thanks
Giusy P.