So the web browser is pretty much a mini operating system. Firefox and Chromium seriously feel like they have the longest compile time out of any package in Gentoo, except maybe Libreoffice. It has tons of embedded packages that it doesn't pull from the system/native (jpeg and png decoders and such). So with a lot of these neat things where we compile stuff or run a Linux kernel in the browser, we've pretty much come…
The obvious answer is that it makes applications portable, which is great. The other key component I think is delivery. You don't ever install anything, it just exists when you ask for it. That is something native applications have never done, and not even something like JVM has done even though it addresses portability too.
It is also nice that it is seamless with the browsing experience, but I have to wonder if it hasn't just turned out that the typical browser interface is a better interface for computing than most popular OS have been? As the OS and browsers start to see parity, are there lessons from browsers that we can integrate into the OS?