the facebook app uses email notifications in order to generate a notification on your blackberry app. ideally, you select it to not leave this notification email on the server. once you get the notification pushed to your phone by that email, you dont need it anymore.
examples.....
leaving them on the server: lets say you get 50 notifications onto your FB app. later on you check your email on your computer. you will see 50 new emails from facebook regarding all those notifications.
its irrelevant, as you already checked them via the application.
deleting them off the server: lets say you get 50 notifications onto your FB app. later on you check your email on your computer. you will see no emails from facebook regarding all those notifications.
its better to not leave them on the server. the FB app will process the email, notify you by way the application notifications and then delete the original. get it?