The iframe embed method loads a entire website from the video host. A youtube embed page is a whopping 561.9 kb huge on top of additional http requests the browser needs to load the iframe(s). So your page is basically loading another website inside your website for every video you embed. This is very bad for performace reasons especially on slow connections. ARVE Pro loads a chunk of HTML with useful SEO data and a thumbnail image instead.
Only when the user actually clicks on a thumbnail the iframe embed is loaded to play the videos. This performance improvement, even when working great for single videos, this becomes especially noticeable when there are multiple videos embedded on a page.