Do you feel the need to offer justifications?
Supposing someone asked why are you wearing a skirt.
At this time of year, a reason could be that the weather is warm - too hot to wear pants.
Or if someone asked why wear makeup. The answer is because it enhances the look of my face.
So what about wearing a bra?
I have read on different message boards, that most women will choose to wear whatever clothes they like, without attempting to look like a man.
But why would a cd wear a bra when there is no need to? I personally don't feel dressed without one. It is just difficult to explain to people who have never met a crossdresser before.
I tend to wear a bra to bed even though no one is going to see me (not even myself). It just feels right. Why I do not know.