From Brian Hill:
MacJanitor is designed to be used on a periodic basis by Mac OS X 1.0 users who don't leave their computers on 24 hours a day. MacJanitor is provided as freeware as a service to laptop and energy-conscious home users. The Unix subsystems on Mac OS X were originally written for machines that were typically never shut off. Mac OS X inherits this assumption in version 1.0, and has many system-maintenance tasks scheduled to run between 3 and 5 a.m. In addition, scripts are designed to run weekly on weekends and once a month in the middle of the night.
If these maintenance tasks are never run (such as on a laptop that is always shut off at night), many log files and system databases will grow extremely large or fail to get backed up. MacJanitor provides a way to run these system tasks at the click of a button. Laptop users could click the Daily button every morning (or every few days), and office workers could click the Weekly button on Mondays.