The folder monitor lets you specify certain folders you want to have monitored for files of certain ages. For example, you may want to remove all files that are older than 60 days in a certain location and move them to another archive location, etc. Or maybe you want to remove empty folders after they are 20 days old. This is where you would do this. In some cases you might set up two different monitoring processes one to delete the files that are older than 60 days for example, and the second process to monitor empty folders and delete them at a shorter time interval. In normal cases the folder monitor on it's first scan deletes the field and on the second scan will delete the folders as well with the next set of old files.
You can specify a custom CrushTask plugin to run as well allowing you to move items off to another SFTP server, or generate emails, etc. You have unlimited things you can do at that point.