OK, I tag the ass of pretty much every girl I bring back to the boat. I like it because it is tighter, warmer, they can squeeze it better, and perhaps a little because it requires their full surrender. I find most girls love anal, once they get past their hang-ups. Like Supra, I have found that when they like it, they really go crazy. Orgasms like nothing ever just vaginally. Position does not seem to matter much, but her on her back and me kneeling in front does indeed stimulate their vagina as well. And I believe I can actually hit their G-Spot from their ass, especially in this position.
If you stick a finger up her ass, and a thumb in her vagina, you will find that there is only a thin layer of skin between the two - not much more than a condom's thickness really. So whatever you do to the ass will be felt, at least some, in the vagina. If you play with it a bit, you will find it is very stretchy too. So if you poke around her ass with your dick, you can hit all kinds of interesting places. But I don't think that is what really gets them going so much.
The vagina has a clit at the top, and a G-spot inside - that's about it for stimulation. The clitoris does indeed have the most nerve endings of any part on a womans body, but it is outside the vagina and is also a small , easily missed item. The anal sphincter, on the other hand, is a solid bundle of supersensitive nerves. In fact, it is the second most concentrated bunch of nerve endings on a female body - and unlike the clit - when you screw it you are actually stimulating the entire nerve bundle. So screwing an ass is a much more intense excitement of nerve endings than accidentally bumping a clit occasionally during vaginal sex.
Once a girl gets over her fear of being "dirty" or that anal sex is a "sin", she can allow herself to feel these sensations. And as Supra has so wonderfully described, it drives them wild. Of course the dense bundle of nerves is a double edged sword. If you don't keep it extremely well lubricated, that wonderful pleasure can quickly turn into excruciating pain. So lube them up good, and they will respond in ways you never dreamed possible!