You don't need the calibration kit if you ride street only. The HP ECU comes with an optimized map for a fully open exhaust (and with map, i mean everything: fuel, ignition, DTC modes are totally different, O2 and exhaust servo's are disabled, and probably some more stuff i don't remember).
The Calibration tool allows you to change fuel mapping too, but thats not the main pro of it. The HP ECU gives you an optimized map for a race exhaust too, which is a very well done map. I havent seen a bike with either a PCV or a Rapidbike output more HP or have a smoother torque curve than one with the HP ECU.
The calibration kit is intended for race teams and gives you the possibility to edit in depth parameters of the DTC (using data logging sensors), enable/disable various functions like fuel cut-off when off the throttle, O2 sensors on or off, knock control on or off, and that sort of gizmo's.