It's attention grabbing, mainly. I live in Brisbane and every time I walk past a Guess-brand shop now, I can't help but notice the enormous posters they've put up, featuring some sweaty, semi-naked model and her boobs (parading various handbags for some reason). It's not a particularly effective sales tactic; mainly because it makes you feel like a pervert every time you walk by, but it sure as hell draws the eye.
It must be more effective than I assume, though, because these days it seems every second shop in Queen Street Mall is wallpapered with photoshopped women. It's the same basic principle for movie posters, game boxes, novel covers, shop catalogues... pretty much everything, really. It makes you look, and sticks in your memory.