I"m the same rookie who brought you "What is an airbox". I now ask you, what is a powercommander, and what does it do? What application does it hold to a stock monster 620 that is never on a track?
in laymays terms the unit plugs inbetween the factory ECU and the fuel injector. There is a wire the senses the throttle position. It then adjusts the signal going to the injector and thus alters the fuel curve. The unit also has a serial/usb interface to allow you to customize the mapping inside the powercommander.
Many claim that monsters ship to the US running a tad lean. The PCIII would allow you to correct this situation. Since you can modify the map for the PCIII as you add modifications: slipons, open airbox, stronger spark, you can adjust the fuel curve. With a good dyno session you can tweak it even further.
If you make no mods I wouldnt worry about it. Just take the bike to the dealer to tweak the ECU to richen things up.
I looked at PC's website, and it looks as though there is a matrix of throttle position vs RPM. Then you can change the number in the cell corresponding to where in the RPM/throttle you want to tweak. What does that number in the cell mean?