If you need some native feature, browser vendors can force you to use a native app and thus go through their app store (think Safari on iOS). There is a large category of apps that you cannot make in HTML, for example anything truly P2P (not WebRTC, but based on real sockets). You can't make an IMAP client. You can't make a friend-to-friend file sharing tool, without a central server, that uses your Facebook and Google contacts to find peers (I've tried). You can't make the browser window partially-transparent or use native looking widgets. You can't burn a DVD. And if someone invents free-floating holograms, you can't add them to your page.
Granted, these examples are silly. The web is now a very capable platform. But vendors have always been steering what you can and cannot do based on business and other interests. It would be great to be able to break out of that, by having a kind of "C foreign function" interface for the web.
It is probably why we all have flat windows 3 inspired UIs now.