When you visit a web page, your browser downloads all content of the particular page as well as common static files like css and js files. And when you visit other page of same website, your browser downloads them again. But if you have enabled Leverage Browser Caching, then all statics files will serve from your browser instead server. Now when you will visit any page of the particular website, it will only download unique contains of the page and static files will serve from your browser. in this way, it speed up each page of a website.