Because a company that is known for raincoats and scarves needs nude shots in its advertising?
Advertisers are bizarre people. For their ads, they already photo-manipulated her face:
http://yfrog.com/05picture8cpxp
The Burberry ad face is the one in the middle.
I would literally not be surprised if they wanted her to pose nude, then to overlay her body with an image of the clothing, so they could control the shadows, draping, etc. more than if she wore it and they modified it after.
Add to this that black and white modeling is bizarre in its own right. First they paint the models with either green or blue foundation, then add several layers of makeup on top of that. Lots of models refuse to do it at all.