When your bike comes from the factory, it is tuned for the parts that are installed. When you change the exhaust system or air filter you can make your bike run lean or rich with fuel which can make your engine run hotter or hesitate when you twist the throttle. In worse case scenarios, damage your valves. The power commander adjusts the fuel and air mixture so your bike runs smoother throughout the power band. It can also reduce popping and backfiring from your after market exhaust.
I mean it is not really a "need" but if you are going to get a slip on exhaust, I don't think it is really needed but if you are going to get a full exhaust system with headers and all, it will probably be a good idea to get a power commander.
You can download maps online depending on your setup or take them to a dyno to get custom maps. I think the hardest part is installing it because you may have to remove your gas tank to get to the wire harnesses at the air filter box area, you can probably get a shop to do that if you are not comfortable. After that, the actual power commander can be under your seat easily accessible to put in or change maps.
