There are a few sexual cravings every woman has: Some kind of penetration, whether that's with fingers or a penis or an inanimate object propelled by fingers is one. Another is the initial lean in of a kiss, lips gently parting for the first taste of tongue. The third? The butt grab.
I don't know how to explain it necessarily. I just know that almost every girl loves having her ass grabbed by her partner, especially as a part of foreplay. And especially when he slides his hands in your underwear to get there. It's an instant turn on—as if you weren't already turned on—that gets you ready in a way that few other things can. And fine, it's not a position in its own right. But it's pretty damn great, wouldn't you agree?