As far as I know a woman may put on fragrance but not a lot.
My question is if a man can’t smell it why she can wear it.
A woman can put on fragrance for herself and her own pleasure, so she feels she smells good or because she likes the good smell of the fragrance. That is fine. However she should not put on fragrance to the degree that foreign men other than her husband will smell it. It isn’t tzniut in of itself because it is attracting in an unnecessary way, (men will know she is around even if they don’t notice her). Besides, a man is not halachically not allowed to smell the perfume of a woman that is prohibited to him. She can however put on perfume for her husband to smell, when they are permitted. Also if she wants to put on a lot of perfume in a place that here aren’t men around, although it is lacking in tzniut to walk around in a loud attention grabbing way, it would be halachically permitted, because no men can smell it.
O:CH 217-4, M:B 16-17.