just as a general history of these sorts of photos there are several reasons for taking them : 1. they are headshots for actors(actresses) 2. they are nude studies (or 'erotica' depending on the pose etc) 3. they are costume studies (these are often the casino dancing girls etc.
there isn't a way to telling the original idea behind this photo being taken as the information has been lost. however it's (to me) an emotive portrait inwhich there seems to have been great thought into captre the girl's beauty and emotion. this would suggest that it was either an art piece or a photograph done for a stage actress
this is a very much a 'why was it taken' reply... but i agree with you, that no matter the reason why it was taken, it looks as though a more personal story between model and photograher (or character portrayed) is just beyond the surface