CVS maintains its own logs, which you can access using the `cvs log' command. This duplicates the information present in the ChangeLog, but binds each entry to a specific revision, which can be helpful at times.
However, the CVS log is no substitute for the ChangeLog files.
In some circumstances, though, the CVS log is more useful than the ChangeLog, so we maintain both. When you commit a change, you should provide appropriate text in both the ChangeLog and the CVS log.
It is not necessary to provide CVS log entries for ChangeLog changes, since it would simply duplicate the contents of the file itself.