Why? Fullscreen mode + auto-hide toolbars + context-sensitive radial menus = one virtual desktop with nothing but web page, navigated via gestures. No toolbars, menus, or borders wasting screen real estate. No multiple browser windows (thanks to tabbed browsing, also hidden and navigable via gestures). No popups (thanks to, er, popup suppression.) Beautiful.
I decided long ago that having more than one window onscreen is overrated, and that toolbars are stupid. The screen should be devoted entirely to content, not application infrastructure, and definitely not to windows half-obscured
by other windows. Sadly, most apps--open source apps in particular--are careless with the number of windows they open. The Gimp, for example, is an interface nightmare. A new dialog excreted every few seconds, usually on top of the content you're working with. Blech. (And I say this as a diehard Gimp enthusiast.)
Phoenix is the first app I've seen in a while that's really compatible with my lifestyle. It makes me happy.