Usually, when someone on the Internet sends email to a non-existent address, the mail gets bounced, or returned to the sender as undeliverable. Even if the domain name is valid, the mail will still bounce if the user specified in the address does not exist on the server that the domain name points to.
By use of a catchall, however, your Freedom account is able to successfully receive all mail that is sent to an address on an associated domain, even if the specified user does not exist. By using a catchall, email addressed to your domain that cannot find a valid recipient—and would therefore bounce under normal circumstances—gets successfully delivered to whatever destination you specify, without returning an error message to the original sender.