The ECU, or the engine or electronic control unit is the brain of the engine, the device that controls how much fuel is injected into the engine under different conditions such as ambient temperature, altitude and so on. Stock engines are designed to work under various conditions, and with various qualities of fuel. Therefore, the motorcycle manufacturer designs an ECU that will adapt to such conditions by opting for a conservative approach that means the motorcycle's engine is tuned for efficiency and emission compliance rather than for outright performance. Enter the aftermarket or piggyback ECU. These aftermarket or piggyback ECUs are meant to exploit the full potential of a motorcycle's engine, and usually are tuned to increase power output by say 5-6 %. Aftermarket or piggyback ECUs are also tuned to make throttle response sharper, resulting in a livelier, fun-to-ride motorcycle. The downside is that they're aggressively tuned, and may not accept all kinds of fuel qualities, and may not work optimally in all kinds of conditions. So, do you really need one? There's no straight answer to this. If you're using your motorcycle on the track, it would be a good idea to use that extra 5-6 % of performance giving the motorcycle a higher top speed. However, do be advised that this will come with the prospect of greater engine wear, and in extreme cases, severe engine damage. If you're riding on the street, and use a piggyback ECU for a sharper throttle response, you could run afoul of the law, and also the manufacturer's factory warranty. So, that's one big downside. The other one of course is the fact that an aftermarket or piggyback ECU could reduce engine life if the rev limiter is lifted. Net-net, aftermarket ECUs are not really required for road riding. They're best used on the track.