Double gates are useful if the entrance is particularly wide. Some driveways are very wide, while others extend no more than the width of a car. In the latter case a single gate would likely be sufficient. In the case of a wide driveway, double gates may be preferred although they are more expensive to install and to operate.
That is because a) you need two gates, and b) you need two drive systems. Two electric motors, two sets of drives and two activation sensors. Double gates can be fitted up so that both motors are started using just one sensor, but the difference in installation costs can still be appreciable between the two.