Personally I prefer natural of any size to man-made of any size. However, I think most the ladies who get them do so because they believe the guys like them bigger.
And despite what guys say, they vote with their pocketbook for larger breasts,
The question is not, "Do you prefer natural 36Ds or man-made 36Ds?", the question is "Do you prefer natural 32Bs or man-made 36Ds?", and most ladies discover--all else being equal--guys pay for more dates with mm 36Ds over natural 32Bs.
I posted a note about au UTR lady a little while ago. I received 6 PMs. The first thing 4 of them asked was "What's her bust size?".
There is also a self-confidence factor for some women, but why do you think smaller breasted ladies are self conscious about their size?