simple anatomy. your wrist cocked at a downward angle(standard contour stocks) causes muscle tension which can prevent proper trigger discipline while also requiring more 'chicken wing' positioning of the firing arm. this not only creates a less comfortable grip but also reduces the amount of pressure that can be exerted on the firing wrist, this means more recoil delivered to the shoulder.
the pistol grip allows a more natural angle to the firing hands wrist, allowing the shooter to comfortably absorb some of the recoil into his or her lower arm and lessening the overall strain on the firing shoulder.
I personally didn't like the pistol grip on my shotgun when I bought it either but after firing it, I find it hard to believe that I will ever consider selling it. it may be a popular design for mall ninjas because it looks scarey but I for one like it enough to bear with all the criticism from the "old farts".
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
I never said half the crap people said I did-Albert Einstein
You can't believe everything you read on the internet-Benjamin Franklin
Bean counters told me I couldn't fire a man for being in a wheelchair, did it anyway. Ramps are expensive.-Cave Johnson.