Because crew members are required to be as pleasant as possible to passengers, they don't usually get a chance to say everything they'd like to say. So, cruise ships keep a log where they write down everything that happens while on a trip. These logs can include something as big as a fire that happened or something as mundane as a comment a guest made. It all serves a purpose.
All of that information helps an incoming crew best prepare and spot potential hazards and how to manage them best. We're sure the log is also sometimes just used as a venting mechanism.