In short, Fetch API- it's a new Promise- based standard for doing AJAX requests.
Syntax for XHR was provided more then 10 years ago (XMLHttpRequest2 - about 4 years ago).
Many things changed, we got HTML5, CSS3, also close to start using EcmaScript 6.
It’s time for new laconic API to do AJAX-requests and interact with them.
And time is come!