A catch-all is used when you wish to have forwarding for your whole domain name. It can also be used to catch all the mail that is sent to an incorrect email address.
For example: if firstname.lastname@example.org is set up as a mailbox and a sender accidentally types email@example.com, a catch-all will allow this email message to be delivered to the address that has been specified as the catch-all.
Warning: setting a catch-all can result in an increase in spam as spammers generally try every possible email address on your domain name. Most attempts will normally bounce unless a catch-all has been set.