Be advised, shooting from a vehicle any faster than a human running pace on a nicely paved, flat road is difficult at best. Take it off road, add unpredictable undulations and it becomes nearly impossible to hit anything unless the weapon is mounted to a gyro stabilized gunsight. Have a few weapons firing at the same target - you 'might' get lucky. (anyone who's actually been in an AFV/APC knows what I'm talking about)
If it really does anything, it helps relieve the passengers of feeling 'helpless' in a potentially large target moving across the battlefield.