Ask ten doctors about their reaction to patients who come to the office with a diary of their medical problems and eleven will groan.
It takes a fair amount of body focus to make someone want to record it on paper. For some, I guess, it fills empty hours. Maybe it's a way of fighting against other signals that suggest he or she is unimportant. If it's on paper it must be true and it must be significant. Rarely is a diary an aid to bad memory. If anything, folks who record symptoms have extraordinary memories. The importance of the information is inversely proportional to the length of the diary.
Harold Yocum kept the most extensive diary of any patient I have ever seen. Harold was a dapper little guy, standing perhaps five feet four. A local retail clerk, he was always impeccably dressed: sharp creases on his suit trousers
Burnside JW. The Diary. JAMA. 1987;258(13):1802. doi:10.1001/jama.1987.03400130116047