(Period) Blood, (Boob) Sweat, and Tears
It turns out that not only men don’t know much about women. Sometimes women don’t know either. I read this question and my reaction was: no, we don’t. Then I Googled it and I found out I was wrong. It turns out that some women do put deodorant in their boob pits (not sure it’s called a boob pit though).
I’m guessing it might be a matter of breast size. If a woman’s breasts are large enough to hang against her skin, wearing deodorant or powder might make her more comfortable. I actually found a list of “best antiperspirants for stopping underboob sweat” on Google. Another solution for underboob sweat is going braless (if you can, again this depends on size). If you can’t, then cotton bras and wearing loose-fitting clothes that breathe is also helpful.