On the web, if your server is compromised it's game over, even if the publisher is not malicious. In app stores, you have some guarantee that the code that ends up on your device is what the publisher intended to ship (basically signed packages). On the web it's currently impossible to bootstrap the integrity verification with just SRI.
This proposal aims at providing the same guarantees for web apps, without resorting to signed packages on the web (ie. not the same mechanism that FirefoxOS or ChromeOS apps used). It's competing with the IWA proposal from Google, which is a good thing.