It's important to download JS-generated data as well, such as something in a Blob. You can't set a HTTP header on local content. The 'download' attribute lets the user "download" that without having to wastefully post it to a server and back.
To make what AshleysBrain said a little more concrete, imagine an image editor program (or a music editor, or a word processor, or whatever) that's written completely in client-side JS. There are starting to be a lot of these, actually. Without a working download attribute, the only way to save your work is to post it back to the server and redownload it. That is a slow, ugly, and unsatisfying hack to get data that's actually already present on the local machine.
Even IE 11 has a way to do this (non-standard, of course, but it does have a way to do it :-). Edge supports the standard way.
As I noted in a different post, though, support for the download attribute has already landed in Webkit Nightly for macOS, and is apparently being worked on for iOS.
