This can be used to send a default message out if you're missing information in your database that didn't populate from your template. If for example you were missing the last name of someone, but you had indicated in your template you wanted to include the first and last name for the contact, the message would be incomplete and may not appear correctly. 
By providing a backup message, you can still send something out generically without having to manually edit the message before you send it out. The BatchLeads software will detect that the information you had in your custom tag was missing (in this example, the last name), and the backup message will be used instead.