And the answer is "I don't know..."
By default Wysija is relying on WordPress' cron system. This system is working but it has some mysterious behaviour once in a while, and it starts suddenly without notice or any clear reason.
From what I read in various forums posts it has to do with some late scheduled tasks which sort of get the whole WordPress' cron stuck and that's when you guys come to ask us questions ;)
People at Automattic created a plugin to sort all kind of wp-cron issues you can get : http://wordpress.org/extend/plugins/wp-cron-control/
So at first we were asking our users to simply use that plugin to solve the problem, but as our plugin grew popular, more and more people started to have the same problem.
Now, since that was a complex process to explain to our users, we've created the Wysija's cron option which seemed the simplest solution for you guys.
1 - Tick the option
2 - Save the settings
3 - Eventually you'll need to set a cronjob but that's not even necessary depending on your site's traffic and your server's configuration.
4 - And that should work!
Hope this makes more sense.
I personally think it doesn't :)