the main purpose of a diary entry is to record of what you did that day.
Updated answer
There are three distinct uses of a diary. The first is to record what you did each day, 2d it can also be used to make notes on future appointment, so that you don't forget them. Like a doctors appointment for example. 3d it can be kept with other diaries to become your personal history.
Ann Franks diaries are good example of how diaries are a personal record of a persons life. They are your intellectual property and can also contain fact and figures of important times in your personal life. If you have an eventful life you my use them to write a book.
Diaries are quite important.
The cast of Entry in a Diary - 1990 includes: Erin Noble as Diary
A journal records what you're findings are
It is spelled Dear Diary.
Doogie Howser ended with a diary entry on his computer every show.
It is usually called a diary or journal entry.
diary entry
Dear diary, < Insert question here>
Anne received the diary on her birthday. She wrote about receiving the diary and her birthday and the problems in Belgium and her life.
reading
Writing
diary entry
Does Harry Potter have a diary? I have never heard about that.