Personal diaries have been written since written language was introduced.
Typically, a diary contains relatively frequent records of the authors life
and events, but it can be about anything.
A diet diary is usually used to monitor and keep records of calory intake.
It can be helpful to keep track of why one is losing or gaining weight.
Dream diaries are kept to keep track of dreams. It can be fun to try to
interpret dreams, and it's often crucial to write it down as quickly as
possible after waking up.
Sleep disorders can be debilitating. Keeping track of sleep cycles can be very useful in their treatments.
Keeping a records of your travels is very useful. How often have you not
tried to remember where you were on specific dates, or when that vacation
was? Keeping track of this enhances your memory of journies.
Fictional diaries tend to be intended for publication. There are many
published works that come in the form of fictional journals or diaries.
Some do pretend to be true, but it's usually fairly obvious that they are fictional.