My friend and I were discussing my interest in fireamrs, when he brought up the topic of pistol grips. "They're so you can fire from the hip and spray bullets everywhere" he said.
I pointed out that that was nonsense. "It's rather difficult to hit anything that way, you only see that kind of shooting in the movies."
Then he asked me what other purpose was there for a pistol grip? I couldn't think of a technical answer for the having a pistol grip.
I'd like to know so that I can tell him and any other anti-gunners who may ask.

Let me try and answer this way.

In 1980, the Israeli's ordered 100 sniper rifles from Mauser - these wound-up as Mauser 66 SP.  Basically these were target rifles with some Israeli mandated changes for their use as military sniper guns.

To the point although The 66' has a thumb-hole stock this stock config offers the same pistol grip functionality as an "AR' just looks different.  The reason is better control and better accuracy.

(On FN's the pistol grip is also a neat place to store your cleaning kit !)

The pistol grip is easier to hand on to if you have a wrist injury.  

With a pistol grip it is easier to use different size grips for different people's guns.

With a pistol grip you can (when the law allows) have a folding stock so that the weapon is easier to store and carry.

also makes it easyer to get a good grip on the gun

Because it looks cool, of course .  Why else would they be included in the ban?

It's also more comfortable to shoot and improves trigger control when shooting from different positions.  Just relax you hands, close your eyes and put your arms up like your holding and imaginary rifle.

Open your eyes and look how your hands are positioned.