The catch-all account is automatically provisioned when the service is created. It is the fallback destination for any email that does not match a specific user in the system.
Once users and aliases have been defined their messages will be delivered to their individual mailboxes. The catch-all is helpful if you have forgotten to create an alias in the system. The downside to simply using a catchall for all of your mail is the intermingling of messages destined for multiple users.
The default catchall account is firstname.lastname@example.org