I initially started using it because I found I could not control multiple screens well with gnome and friends. Moving windows between screens and workplaces, moving workplaces between screens, arranging windows.

Then I got addicted to the speed of flipping between windows and not having to fiddle with layout. Now I only have one screen but it is always the first install on a new Linux.

Nice small and configurable window dressing and i3bar were very nice also.