Please forgive my ignorance, and only I ask this because I don't understand the real advantage behind it, except that they "project" a very nice, yet noticeable impression.  But I have experienced the pleasures of women with natural, silicone, and rock hard breasts, and found that the rock hard breasts are kind of a disappointment while indulging.  Granted, they can present a nice appearance, but when the sports bra, or Lana Turner sweater comes off, and you're engaging with them, it reminds of holding to baseballs encased tightly in leather bag.
I don't mean to downgrade them, I just want to know is there any other reason besides the look.