Mail forwarding:
It is not necessary to setup both an inbox account (cPanel > Mail > add/remove) and a forwarder (cPanel > Mail > Forwarders) to forward emails. If all you want to do is forward emails, make sure to NOT setup a mailbox as well. The mailbox can eventually fill up if you do not clear it often enough. When it does, it will send a bounce message to the sender, and will also forward email to the desired location, which can be confusing if you forgot you setup an inbox (now full) and a forwarder (operating like normal).

If you require a forward, but also want to store a copy of the email on the server, then you may setup a POP account as well.

Forwarding is not recommended.
1. Why forward email and answer it from your ISPs address when you have the capacity to use your own address? This improves customer confidence, and re-affirms your brand name, or makes your domain more memorable. you cannot reply from a forwarder, only from a mail account.
2. People turn on forwarding, then when they get spammed, it gets forwarded to their them@their-ISPs mailbox (e.g. Then, that person reports the spam. So what is the problem? When that person reports the spam to their, essentially they are turning themselves in for spamming themselves. Spam hits our mail server > forwarding > spam goes to ISP inbox > customer reports spam > ISP blacklists customers domain IP address from sending email. if you are not paying attention, you can easily turn yourself in for spamming, even though you had little to do with it.