I know a lot of women who do this. Maybe it is regional? I live in the South.
It's *supposed* to leave one dry and fresh. I used to do it, only occasionally, until I found out how dangerous talc is. I haven't done it in years and years, though. I have never sweated profusely; by the end of the day it was pretty much normal down there, as far as the powder.
My cousin died at a young age of ovarian cancer; it may have originated as cervical cancer, I'm not sure. But I found out later that she practiced this religiously. It was very sad; she left a husband and little ones behind.
Cornstarch is supposed to not be dangerous, but my mom insists that the talc powder "smells better"
and I had to work hard to keep her from using it on my daughter.
If powder "must" be used, they recommend only a small amount of cornstarch in the bottom of the diaper, not on the diaper area, and not shaken in a puffy cloud where it can be breathed in. I also read that talc is very similar to asbestos