How about a consistent, reliable implementation of modern web standards? If you had to write a mail client in Java, would you rather target Java 1.5 or the most recent version?
Many small (and large) features together make a huge difference when it all adds up to providing an implementation of modern web standards that is consistent with other browsers and with modern standards. It's a huge burden to always be checking "is this supported in IE11 or is there a polyfill we can use" when wanting to use a native function or object (not to mention features that can't be polyfilled or transpiled, like ES6 proxies).
