External JavaScript will help you speed up the download of your Web pages. You might think that it wouldn't matter - after all, if a Web page needs the scripts, the browser will download it. And yes, if your customer only ever sees one page on your site the scripts will be downloaded.
But once they're downloaded, they are cached. Once your scripts are cached, each subsequent page that is downloaded only requires the browser to download the HTML.
Placing JavaScript embedded in the page also requires that each page be maintained separately. So, maintaining your scripts in embedded files, you help your customers and keep your maintenance work down.
