There is a slight shift in the narrative with the story of Adam being told through hiS abandoned notebooks.
It probably helps to get some distance from Adam because he has become more and more miopic and a trouble to his girlfriend michele.
For the reader it also works using notebooks because it condenses down days worth of activity. before that he was following the dog round the town and brooding on a life of complete isolation.
You know that he is not going to be able to maintain this life for much longer.