The advantages of Promises and jQuery Deferred

The good point

The callback can be defined later on and fired even if the original promise already completed. When calling the promise.done(function(){}) it will fire even if the deferred is already completed - it will just execute right away.

The bad points

Performance.
Use it only when you have async behavior (which happens very often in Javascript). If your code is synchronous, do not use promises it is overkill.

Example of usage :

jQuery AJAX methods

var deferred = $.get('control/Get_SystemInfo.php', function(response) 
{
   
});

(...)

deferred.done(function()
{
});