Google is laying the groundwork for progressive web apps as the most recent build of Chromium for Android was discovered to have a flag for enabling webAPKs. For those interested in grabbing the latest build version of Chromium for Android, Google has made it available to download but they do note that these sorts of builds can be "tremendously buggy," and instead recommend that people either check out Chrome Canary or Dev Channel builds of Chrome for an experience closer to a more stable build of the app. While there is no indication of how long it might be before progressive web apps are brought to the forefront and are more mainstream, the presence of a flag to enable them in the latest Chromium build suggests it might still be a ways off.
While web apps are already available for use on both Chrome for Desktop and Android, progressive web apps should be able to deliver a more powerful web app. In addition to this progressive web apps should be considerably more useful to end users. End users wouldn't be the only ones who benefit, however, as they also enable developers to create web apps more simply, and more web apps due to increased efficiency and decreased difficulty in the creation process means more choice for the end user, and potentially more end users for the developer.
It might be hard to fathom right now how much more useful progressive web apps can be, but imagine a web app which gets more powerful over time the more that you use it. This could potentially mean that it runs faster and smoother, or that it learns how to service the user more efficiently over time too. Progressive web apps can also be used across different OS platforms making them versatile, and with the ability to package them into a webAPK, users are able to open up Chrome and add a shortcut of that web app to the homescreen on their device. Progressive web apps packaged into a webAPK offer a native app-like experience complete with a Material Design UI and all, with speed of functionality being what sounds like the main benefit.