It follows the "action when event" paradigm.
For example you may want to "send an email when a user fails to log in", or you may want to "zip a file when it is uploaded".
When you add a new even handler you have to choose the script you want to run, and the event you want to trigger the execution of such script.
The Priority value is used by Syncplify.me Server! to decide the script execution order, in case two or more scripts are associated to the same event.