Because it looks better in fullscreen, with acceleration, and you can save it if you want to keep it. Totem now with a "Save Copy..." menu item, and a playlist parser that knows about video websites.
The ever present Xan is demo man.
You'll need quvi (for its library) and the master of totem-pl-parser (that should even work with older versions of Totem) for the video website support. The "Save" menu item lives in Totem master, scheduled for GNOME 3.