Email forwarders can be used with or without a POP email account.
When used without an email account, a forwarder can be set up like so:
email@example.com --> firstname.lastname@example.org
In this case, mail sent to email@example.com is delivered to Bob's email account, even though the firstname.lastname@example.org account doesn't exist on the server.
If there is an account for email@example.com, the email is delivered to this address as well as being forwarded to firstname.lastname@example.org.
Multiple forwarders can also be set up if required e.g. forwarding a generic email account to two or more people.