Video Lightbox
Displaying a video in a lightbox window has a lot of advantages - especially when using hosted video like Vimeo and YouTube. This video types download a whole bunch of scripts on page load whether the visitor wants to view them or not.
Loading Vimeo or YouTube in a lightbox prevents this happening until the visitor decides to watch the video.
Appearance
Although Vimeo has a better looking interface, YouTube videos really spoil a page design if seen on page load so using a lightbox makes sense for those who want a professional look rather than cheap and nasty.