Let's take a look at an example:
alert("This Square is now Hidden");
Here, we have a script that hides an element with the click of button. Once the element has been hidden, an alert appears.
Note that the alert only appears once the hide animation has completed. This is an example of an animation that uses the callback function.
Now let's see what happens when we remove the call back function from the script.
We can see that the text is hidden, but also that the alert appears at the exact same time. The events run simultaneously, which is undesirable.