It is probably also used for the black box built into most of the newer cars. I don't know to what extent it is used on the Volvo but I have seen it used in court as evidence to indict a driver. The black box records the last few seconds of events right before deployment of the air bag. Records speed, time and speed at which the brake was applied, and some record whether the passengers/driver were wearing seat belts. This device may have some safety benefits, but the intrusion aspect is scary. The particular case had the driver testifying to driving at the speed limit but the black box, which showed a much higher speed, was used to refute the driver's testimony.

Another possible use is to prevent engagement of the automatic transmission until the brake is activated.
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1998 Volvo S70