Repeat visit, of course :) The service worker can't do anything before it's installed (during the first visit). The browsers' network cache is great, but IIUC the browser still performs those 'if-modified-since' requests, doesn't it? Additionally, depending on your server setup and CDN, some assets may have a short max-age value, meaning they're reloaded more often than needed. With service worker, the website can create its own cache and have full control over it. In other words, the site can guarantee that its assets will be loaded directly from the browser's memory.