In 2012 1,031,631 people became permanent residents of the United States [1].

Obviously there are paths to citizenship. But some kinds of temporary visas don't offer a path to citizenship (like intercompany transfer L visas or F student visas) and this program could be useful to people on those visas if they'd like to stay.

[1] http://www.voanews.com/content/us-issues-million-green-cards...