There are many functions your web application could perform if it had someone always waiting to click on a link at the perfect time. Revalee can allow your website to double as your backend processor. Not all tasks are appropriate for a web application. Long running processes are a bad match. However, most out-of-band tasks are relatively quick and use the same database technology as the web application. A scheduled callback makes it easy to write up another page/handler/action to perform a task that would otherwise require an entirely separate application to launch on a dynamic schedule.
See the details here: REVALEE - Scheduled Web Callbacks.