The benefit of reducing loading external or third-party scripts and media files like Javascript files, fonts, gravatar, etc. is that it reduces the amount of time the browser takes to make a DNS look-up. For a page to complete loading, the browser has to make a DNS look-up to each domain (to request the external scripts) which negatively affects page load speed.
Examples of common third-party JavaScript files used by many websites are Google AdSense js, Google Analytics js, Google Tag Manager js, etc.
Important: It is not advisable to download and load third-party scripts internally because you will have a hard time keeping up with security updates. Only reduce third-party scripts that are not necessary for the functioning of your website.