Offline capabilities allow a website or PWA to load pages, images and data even when the device being used has limited or no network connectivity. This capability allows a web site to load quickly and reliably in all situations, and is applicable to both PWAs and standard web pages.
For a PWA, it is critical that the app loads quickly, and that it can load and display regardless of whether the device it is installed on has network connectivity. But because PWAs are just special webpages, the expected reliability can only be delivered if the PWA's page and resources are offline capable.
Standard websites can also benefit from offline capabilities. Even if a website contains a lot of dynamic, time-sensitive content which must be fetched anew each time a page is loaded, ensuring that the website's static elements are offline capable means that part of the website will load and display, and instead of presenting a blank page is able to control how missing elements are presented until such point as connectivity is restored.