I thought the answer was pretty obvious and as stated by a couple of posters ie. To avoid being seen by nosey neighbours, especially if she is dressed provocatively. But then there is the more cynical and paranoid view, which is to trap you!!
I honestly wonder why some guys actually punt at all. It appears they always have crap punts; fear for their wallets, privacy or their lives. In my experience, albeit relatively little compared to some on UKP, I have never felt threatened, but that's maybe because I am a bit more selective about which WGs I see. I'm never likely to meet a pimp because I stick to independents.
Personally I like the suspense when the girl opens the door. Even when I see a regular I like to be surprised when I see what she is wearing.