The catch-all option is a pretty useful function, but quite underrated.
The email addresses can be split with two parts: local-part and domain name. For example, if we have the email address email@example.com, the local part is "account" and the domain name is "mydomain.com".
Suppose you are an owner of a domain name. If you want to route all the incoming emails to firstname.lastname@example.org when any local part is used, you have to configure the email@example.com as a catch-all.
This way, all emails sent to not existing local-part will be delivered to firstname.lastname@example.org.
This way you never miss an email even if someone misspells your local part. The side effect could be a lot of spam in the catch-all email address, so use it carefully.