If you use jQuery in development, you've almost definitely used $(document).ready(). $(document).ready() is a wonderful function that lets you run Javascript code as the page is ready to handle it. But one potentially frustrating aspect of it is that functions are executed in the same order they're passed in, and jQuery doesn't let you choose which functions run first.
Usually, this is fine, but every once in a while, you really want your function to run before another function. For example, if your page is running jCarousel or Google Maps, or any other library that changes the markup of your page, you might want to do some processing before they get a chance. With the traditional $(document).ready(), you'd be out of luck.