More than the "force download" aspect, it's mostly useful to specify a filename when linking to a download where the filename can be strange or non-existent, like a data: url, or some remote resource you don't completely control.
I'm pleasantly surprised to find out it now works correctly on Firefox. It was Chrome only last time I checked.
Surte, but one cannot underline enough the importance of relevant and correct examples. May I suggest something like "<a href='verybiglogoftheday/' download='log-2013-12-11.txt'>"
Yes, look at my example (http://jsbin.com/utugex/1/edit), the href is pointing to an .html file, so in the normal case the browser will display the content of the page but with the download attribute the browser will be forced to download it as "myfile.pdf".
