Different people write diaries for different purposes. There are certain professions like legal practice, where dates are very important as the cases are heard on pre-fixed dates. Similarly, the professionals who need to fix appointments e.g. doctors, dentists etc. have to essentially maintain diaries. In cases of even more busy professionals and highly placed bureaucrats etc., private secretaries maintain such diaries.
Many housewives maintain diaries for keeping milkman or washerman/laundry wallah records, besides such other routine type of household matters.
During my service period, I maintained diaries regularly in which I used to record important data regarding official activities for preparing reports later.
Regarding maintaining private diaries, care should be taken to not to record too intimate details. Nobody knows as to what repercussions such details may make later, even after many years in the life.
There is perhaps no fixed criteria for writing private diaries. It is totally up to the author to choose as to what to write and what to not.
Let us encourage each other in sharing knowledge.