Superstition: That it will "toughen" up their nipples.
(Like massaging the perineum to prevent tearing, some folks have a hard time truly trusting nature/God has it all figured out.)
We don't need to prepare our bodies, we need to become aware of how to better work with them and to avoid issues, interventions, and complications so that we can ALLOW our bodies to do what they're already capable of.
I totally agree with doulala. It's an unnecessary step that people take that they believe will help with breastfeeding, either to stimulate milk production or to toughen up the nipples, but over millions of years, the body has adapted to breastfeeding, and we don't need to take these steps, which can actually INTERFERE with breastfeeding. Best to let nature take its course.
Yeah, I was advised with my first to "prepare" for breastfeeding by rubbing my nipples with a washcloth, vigorously, as the time came near, but in retrospect I think my (male) doctor was just being a bit of a pervert. :P I've breastfed 6 kids now and it is not helpful or necessary to "prepare".
No amount of massaging can prepare your nipples for the amount of suckling that your baby will do. The nipples are sore at first- any part of your body would be site if it was sucked on for hours a day, everyday. But nipples are strong! They toughen up and the discomfort vanishes after a few weeks:)