That is a really great question, beru! I wonder if it is another one of those things for adult convenience, rather than for the good of the baby. Especially since FF babies have to drink more and more formula to be full, and it would take twice as long to feed an 8-oz bottle of formula than a 4-oz bottle. So faster-flow nipples would cut down on the time spent feeding the baby. I don't know, I'm just speculating. But if so, that's a really backwards reason for doing it!