To her, that mask means so many things. A reminder of what Charlie had taught and shown her during the time he had taken her under his wing. It is a means of helping her to find herself-- to look in a mirror and see nothing but a blank face staring right back at you allows her to look at herself beyond a recognizable face. To see and understand who she really is, and in doing so, not only answering her own questions but to discover new ones as well. --Plus, it's pretty amusing when a criminal freaks a bit when they see that you don't have a face. A small bonus, really.