It's important to cover because it will help the effectiveness of your web page.
Not so much for the website visitor, but to the search engine spiders and how easily your web page code is spidered.
When a search engine visits your page it's looking to find out anything about your page or website it possibly can to determine if it has usable content and if the information is relevant to the keywords you have listed.
It was used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.
Microsoft has its own version called Jscript.
An example is having the date on your sales letter page always showing today's date.
Or, a pop up window that opens up when you click a link.
This doesn't have to happen and in most cases shouldn't happen.
The web browser reads this method exactly as the other method but it provides much cleaner "code" for you to work with and allows the search engines to see more of what you really want them to spider.
js file such as yourcode.
Then, in the same location you removed the code from, you would add a statement to the page like this: It's important to make sure to have the "type" listed to make it valid code, but it is most important that you have the "src" (source) path correct.