From http://rails.co.za/articles/2006/06/03/using-railscron:

RailsCron, by Kyle Maxwell, ‘is a way to run background tasks using your Ruby on Rails environment.’ You could arguably do what ever you want to do in the background via running a crontab that executes the script/runner, but RailsCron enables you to do it all from ruby code, and seeing as it is an ActiveRecord object, you can manipulate it as such from your application.

Rails Cron isn't the be-all and end-all of background task automation, but it can provide a useful crutch to lean on in some situations. If you need to send regular mails to users, etc, give it a look.