Think of this situation: You're in an un-armoured civilian car, going around a small village as a Blufor, while a friend drives. Suddenly, an enemy jumps out of a corner and opens fire directly at the car's window.
You're now completely defenseless, just a sitting duck for the enemy (Unless you jump out directly into his bullets). But, what if you could duck down quick enough (Maybe by pressing CTRL + S) to protect yourself from enemy fire? Maybe even if the player has track IR, do it physically.
This was clearly implemented into helicopters. Pressing CTRL + Q/E makes you tilt to either left or right. Why not also for vehicles?