For MTF crossdressers or transgender individuals, breast forms can really help with “passability”. Not only do breast forms add weight to your chest, which in turn naturally causes the shoulders to settle back into a more feminine position, but they also visually add curves to an otherwise masculine frame that can help a person blend in better (or stand out) as a female.
For some people it might be strange to think of breastforms has helping someone “blend in”, especially when you try some on for the first time. If you’ve never worn forms before, or aren’t used to having the extra weight on your chest it feels like they really stand out – but that’s not necessarily true. If the goal is to look like a natural female, then it’s important to mimic a natural female look as much as possible. While there certainly are genetic women in the world who are completely flat chested, the majority of girls are not. So having a shapely chest is as natural as not having a beard. So even if it feels strange at first, chances are you look a lot more natural with them than without them.
My personal objective and preference in having breastforms is to “blend in”, not to stand out. But certainly there are those who would be more interested in standing out – and for those people there are large forms that scream “HELLO!” Since my objective is to develop are more natural look, I have chosen forms that should hopefully compliment my frame, and look normal for what one might expect for a genetic girl in my family.