What is the purpose of the JQuery object promise?

For a couple of years, I was dealing with a part time problem where some DOM elements loaded with jquery html function were not accessible immediately after the function call (The behaviour was a bit like executing a function without waiting for the document ready event). Even though .html() is suppose to be synchronous, this SO answer (and also this one) suggests using the promise of the html function:

$('#divId').html("<h1>hello</h1>").promise().done(function(){
    //Callback
});

A promise could make sense in such context but what intrigues me about this promise is that it will also be returned with every jquery's object:

$('#divId').promise().done(function(){
    //Will also be called...
});

Since I couldn't find anything about it in the documentation of the html function, I was wondering what was the real purpose of this promise and if it is used correctly in this context.