The battery of a car puts out about 12.6 volts. In order to charge the battery, the output of the alternator is over 14 volts. If you disconnect the battery, there will be a spike in the voltage from the alternator which will then send more that 14 volts to all the electronics in the car. This may damage these electronic components.
Even in the old days before electronics, when cars had a generator, disconnecting the battery would cause a voltage spike which could damage the ignition points and the radio if it was a transistorized unit and happened to be on.