My reply was an instinctive one... I didn't give it that much thought until you asked "why"?
For me personally I think it's first of all an extension of the thing you get when you walk into a bar with a beautiful woman. Obviously other men (and women) will stare etc. The turn on for me (and I know plenty of other guys who'd totally feel the same) is the affirmation that the woman i'm with is indeed desirable. And of course the implicit affirmation that I myself am also desirable as, after all, she could have had any of those other guys, but she chose me.
And ofcourse you get the added bonus of saying "stare all you want dude, but guess who's getting laid tonight, and guess who goes home to have a wank".
So back to the pussy stuff... yea, I suppose it's the same basic mechanism. Just a bit more "extreme".
Second thing I can think of is breaking the social norm. I suppose social convention assumes that your wife/gf's pussy is between you, her and occasionally her gynecologist. If she shows it off to the rest of the world it's a clear sign that she doesn't let social convention inhibit her own sexuality, and that she's obviously sexually adventurous.
Like I said, this is what I personally think is the turn-on for me. Other boys must speak for themselves if they feel inclined to do so.