Using Javascript Libraries

I'm missing some fundamental concept regarding the use of javascript libraries in IDEA 11. I have a project with a Web module. I was looking around at the lib Javascript Libraries section in the Settings dialog and do not understand what they are used for. I have clicked the "Download from content delivery network (CDN) and install" link and installed jQuery, etc. I see where I can specify "Usage Scope" and what libraries to include...

However, I'm having trouble figuring out how and why to use this feature. As far as I can tell, I still have to copy the .js files to a folder local to the web module. Is it just so that autocomplete works?

As I said, I'm probably being dense and somethings not "clicking" for me....