The way I see it, the advantage is speed through efficiency of motion. People who use Vi or Vim these days tend to be expert users. Design for expert users is totally different than your average consumer. Expert users will generally take the time to learn and remember complex shortcuts if they have enough of a speed payoff.
I personally use Vim for all my development, because it's that much faster. Moving your hand back and forth between the mouse and keyboard takes a lot more time than you think it does. The same applies to moving your hand to different parts of the keyboard. By putting the "arrow" keys right under your fingers, users don't need to move to access them.
It's almost always faster to keep information in the mind than on the screen. The same works for the keyboard: it's faster to just remember that j is down than to move your hand to a separate part of the keyboard that provides a visual key mapping.
Vim is terribly complex, but it's specifically designed to minimize keystrokes. The layout of the specific keys is an arbitrary mapping carried along from the early days, but learning that mapping is something you just do once and move on. It's not intuitive at all, but that's not the point of this design. The goal is speed.