This plugin caching all js file to local cache directory, you may also apply gzip or deflate to this folder, using your webserver (.htaccess for apache for example).
It also minify response time for each external JS files because no more need to DNS request it, establishing connection, receiving data from antoher server. Your server process such files and send it to the client in one keepalive connection.
This increase load and render speed of your joomla site, reduce bandwidth (especially if you enable gzip or deflate for cache folder) and reduce bounce rate on big web-sites and portals.
This plugin also support timeout connection for caching each js file, you can adjust this value in plugin settings. If timeout was reached this current js file willn't be cached.
Not forget to chmod /cache/ directory for writing and enable the plugin.