Schedule Actions add-on makes use of the WP Cron functionality to schedule the actions. WP Cron is included by default on all WordPress sites.
In addition includes built-in support to the Automattic’s Action Scheduler as alternative way in case you are facing issues with your site’s WP Cron.
WP Cron or Action Scheduler?
There is not so much difference between those options, both ways work correctly and in a similar way.
WP Cron is included by default on your WordPress, but as every WordPress built-in functionality can be extended or disabled externally, for example, through a server configuration, through your WordPress configuration or through a plugin or theme.
Is not common to facing issues with WP Cron, but there is the possibility that it gets stopped for several reasons, for example, hostings with lower resources commonly disable WP Cron to reduce the resources consumed by your site, or “optimization” plugins could disable WP Cron too for the same reason.
In case the scheduled actions of your automations never gets executed, you can choose to install the Action Scheduler plugin (already included on Automattic’s plugins like WooCommerce) to get it fixed temporaly until you look for a way to get your WP Cron fixed.
Or in case that you prefer to use the Action Scheduler plugin to handle scheduled tasks on your site, is completely right.
How to meet if WP Cron works?
A simple test that you can perform to check if WP Cron is working on your site is to schedule a post and check if the post gets published automatically in the date you setup.