Little ArtistNaminé is frequently seen with a sketchbook. I would go as far to say that her sketchbook is actually very important to her as she is often shown clutching it close to herself and treating it delicately. Her pictures usually depict events and relationships between people involving Sora and Roxas, so at first I was under the impression that her drawing was a way of her utilizing her powers. However, when piecing together Sora's memory later on, it would not appear that she actually needs to draw in order to do anything. It seemed more of a hobby to me, especially since she implanted a memory of her always drawing when they were little in Sora's heart.
I think that drawing actually served as an escape or an emotional outlet for her. With no one to talk to or rely on, drawing pictures probably became a way for her to forget what she was doing. Drawing was a way for her to forget how painfully lonely she was. Maybe, she could even pretend that what she was doing was all in her imagination rather than being real. It could have very well been a defense mechanism to protect her from being overwhelmed with guilt, fear and depression. As time goes on, she is shown less and less with her sketchbook and without her drawings around her. Of course she probably has less time once she is free from Organization XIII as well, but I partially think the need to escape into her sketchbook becomes less as she finds more of a purpose in existing.
A second thought that I had relating to her sketchbook would be that since she came into existence with no memories of a past, maybe drawing the memories of her in Sora's memories made her feel as though she had something to look back upon fondly. She certainly was not creating pleasant new memories of her own locked away in Castle Oblivion. Even though she knew they were fake, she may have found some escape in imagining them to be real and thus drew them on paper.
Of course this is all pure speculation on my part, but I find it hard to believe that Naminé would not have gone crazy being alone for so long on top of being forced to do something she truly did not want to do without some form of escape or relief. There is no direct statement that she uses her sketches for anything in particular. Therefore, I think it could be a logical conclusion that she may simply draw as a way of emotional expression.