This has been designed to return an improved economy in turbo diesel engines. By recalibrating the torque limits on the engine, at lower rpms you will benefit from additional performance plus improved economy. At higher rpms the software will encourage shifting up through the gearbox, to keep the engine running in its optimal window. This is a popular choice with large fleets where the extra mileage per gallon counts.
Diesel engine remapping can save you money on fuel every time you drive; normally remapping saves 8-15% (dependant on driving style) but with greatly enhanced driveability. An average of 10% saving soon mounts up. If you're a commercial driver doing long mileages then the remap will pay for it very quickly, or if you tow/carry heavy loads, the benefits are quickly rewarded.