I can't imagine asking for or sending nudes, but the point of it seems fairly obvious--to move the interaction quickly into sexual territory. And for those who see dating as having no purpose but finding sexual partners, it makes perfect sense. Once someone has undressed for your eyes in a picture, it's a much shorter step from there to doing so in person.

Anyone who asks for nudes is making it clear that their interest is limited to activities that can be done unclothed. Which is a good thing, as it lets actual relationship-seekers screen out those prospects, and lets players get on with their game.