I misspoke. I meant that it supplies power to all the accessories that are hooked into it as one unit and didn't mean that it supplied different amounts of power. Instead of having several "hot" wires that supply power throughout the harness, it's all one. So, yes, it is a power distribution box. Only one "hot" wire in from the battery that supplies power to all the lights, ect.
As an example, you would have 1 wire from the horn switch that goes into the M-unit and 1 wire that goes out to the horn itself. (not including ground)
If you add a M-Button, you have all the wires from your handlebar switches (i.e, headlight hi/low, turn signals, horn, starter) go into the headlight bucket as normal. But then only 1 wire back to the M-unit. The M-Button is also small enough to fit inside the handlebars for a much cleaner look.
So, 1 wire from the headlight back to the M-Unit (under the seat) and 6 wires that run back to the headlight, signals, horn, ect across the top tube.
Only 7 (input included) wires running forward across the top tube, from under the seat, and 3 wires running rearward to the tail light/rear signals . Not bad.
Oh, want to disconnect the headlight? Unplug it. Want to move the entire unit to another bike? Then move the whole thing.
Revival Cycle has some good videos about it's capabilities.