Aziraphale keeps a diary, in part, so that Crowley can show it to him if Heaven ever takes his memories
I think that Aziraphale has written a 603-volume-and-counting series of secret diaries about his experiences on Earth and his relationship with Crowley not just because he loves humans and books and paper and ink but because he's well-aware that Heaven will often take the memories of angels as punishment if they are caught committing acts Heaven finds subversive. You just know he's hidden them all in the walls of the bookshop and told Crowley to show them to him if he ever has been around Heaven and then suddenly doesn't seem to know who he is. The diaries are Aziraphale's attempt at making sure that he always has a way to at least know, even if he couldn't ever fully remember, so that Heaven can't ever completely take their love from them.
















