I think it's mainly because it feels naughty, and the anticipation of it is really sexy, to not know when the next one is coming, or whether he's just going to caress you instead...
I've recently discovered it and I think that every woman can do it... you just have to look up the right method.
the orgasm is normally so intense and obviously wetter=better too :P
Love a man in a suit, especially if they have a muscley back...