But unlike normal OS-level window managers - even ones with very nice features like Exposé - Tab Candy gives you a single overview of all your "windows" and the sub-windows (tabs) nested inside them. And you can interact with the "windows" and change their contents from the Tab Candy overview, which you can't do from Exposé or similar. And the naming, the simple physics for arranging spatial groups, etc., make this suitable for different use cases than ordinary windows.

Try it out and see. I've been using the internal Tab Candy alphas for a while now (I am a Mozilla employee) and it really adds some useful new possibilities.

In fact, I would love to see an operating system with a window manager that had more of Tab Candy's features. (GNOME hacker Jeff Waugh agrees: http://twitter.com/jdub/status/19052579869)