Another (popular?) use is for SPA or JS-heavy pages to render themselves for search engines. Crawl the site with PhantomJS. For each page, give it a few seconds to do XHR and render things, then save the HTML snapshot. When you get a request from Googlebot, serve the HTML snapshot instead of the app, which Google apparently still cannot handle. Ta-da!

(And piss off people who hate JS "apps" or still desire the HTTP/HTML document ideal, for better or for worse.)