A question that often pops up when people need to learn the command line interface is “why?” One of the reasons is that if you know how to perform an operation by hand using the CLI, you can probably do it using cron or at. These two utilities allow you to schedule tasks to be run periodically or at a particular time.
Being able to run a task periodically or at a certain time is very helpful. For example, log rotation should be done daily or weekly, depending on the nature of logs. Most servers also have some form of back up program running periodically.