There are many reasons to use Google CDN when adding jQuery to the front end of your WordPress site. One of the these reasons is that many of your visitors may not need to download it at all. When your visitors browser sees a Google CDN hosted copy of jQuery and if available will load from their browser cache negating the need to download the file again. Adding this snippet to the functions.php of your wordpress theme will register and include jquery from the Google CDN.
You may want to take a look at Googles hosted libraries to see what else is available.read more