Similar to a tachometer in commercial lorries, this measures data such as speed, throttle applied, rate of acceleration, suddenness/degree of braking etc.
With a bit of bluetooth conversation you could also record data such as speed limit on current road, position of other cars, road signs etc.
this information it would be possible to see who had cut people off, been speeding, driving too close to the car in front, etc. This could then be used to confront the driver and explain how he's/she's driving like an arse.
I'm aiming this mostly at company car drivers; I can't imagine most private owners would want one of these...