Log files are where messages pertaining to events that happened on the system are collected. Log files for a *nix machines are often managed by syslog or a syslog replacement or by the particular application itself.

Log files are useful for analyzing the system as log files contain both indications that the system is operating as it should as well as signs of hardware malfunctions, software mis-configurations, or an attempted exploit of an application.

It is often useful to analyze these log files as they are being created; to perform log analysis in real-time. There are several popular log analysis applications that can do this (In my opinion, Logsurfer is the best one out there.)

It is often good for a systems administrator to be able to provide summaries of the data contained within these log files for the suits.

System administrators who are also psychic have no need for log files.

System administrators who are not psychic, but would like to appear so, may be interested in log analysis application such as Logsurfer and Swatch.