Bras have always seemed to me to be something of a symbolic halter - i.e. another subtle (or not so subtle) reminder that men are in charge and women ought to be undert their control.
Perhaps my view will seem very odd to some. Perhaps it is odd. I don't know. What I do know is that there are many insidiously conditioned - this is your place - pointers in society, not only in terms of gender but in many areas, not least being that of status or class or knowing.
Of course, I can understand that for many women, perhaps even for most, some support for their breasts may be very welcome, particularly if and when they are engaged in vigorous activity. So I certainly don't decry the wearing of bras or suggest that they are of no value.
The critical issue, in my view, is whether a girl or woman has made a conscious choice to wear one and, preferably, a choice based on her needs and comfort and predisposition, rather than to please others or because it is the 'correct' or 'normal' thing to do.
That being said, I'm a male so what would I really know about this issue. I can find cleavage attractive or almost repugnant; far more than I care for it is the latter.
Such questions are part of a much broader and significant issue, in my view. That issue is the preoccupation with body image that permeates our society today. However, that discussion is for another time, perhaps.