it's both. it does feel awesome, but it is the slutty factor too.
it also makes an average dick seem huge, and it's cute/funny to plump out the cheek of a pretty face like a cartoon

seriously, it's a whole host of things and subtext behind it i'm sure, just like a facial. i've also had girls say they liked rubbing it on their teeth that way.
...or at least they said they did..lol

and any girl that makes the effort to do something a little silly always earns points