A ship´s log can be used for a number of different purposes.
Today's ship's log has grown to contain many other types of information, and is a record of operational data relating to a ship or submarine, such as weather conditions, times of routine events and significant incidents, crewports were docked at and when. It is essential to traditional navigation, and must be filled in at least daily.
So ... the purpose is to keep you sailing nicely through out the lessons without any trouble or getting lost.
Best luck sailors!