cPanel offers two ways to forward email:
1.) Forward incoming mail to another email address, keeping a local copy of that email
2.) Forward the email without keeping a local copy.
To forward email, and keep a local copy the email account must be set up in cPanel as its own login account, the normal way. But if you do not wish to fill up your disc space with thousands of accumulated emails, forego setting up an email account in cpanel, and just set up the forwarder.