The diagnostics page exists to make it easier for your to debug any issue regarding your server and WordPress environment. It checks all necessary server configurations, packages, and configurations that are required to use Simply Static as smoothly as possible.
As you can see it does not only checks WordPress-related features like WP CRON or pretty permalinks, instead it also checks if Simply Static can write to your database and that you have at least PHP 7.2.5 and the necessary packages installed on your server.
When you scroll down the page you will notice that there are additional debugging options.
Here you can activate the debugging mode which creates a debug log file within the Simply Static plugin folder. It’s extremely detailed and contains pretty much every information you need to optimize your server, WordPress, and Simply Static configuration to make sure everything works perfectly for you.
You can also send the debug log via E-Mail which is useful if you hand over the debugging to another developer or open a support request on my site.