Some folks like the idea of keeping the same email address for life. It permits a personal and permanent email address (e.g. email@example.com) even as you change internet service providers. You can easily forward to your new provider account (Comcast, Earthlink, etc.) or your cloud account (gmail, hotmail, yahoo, etc.).
When doing this you are now relying on the security of two platforms (host and destination).
Though I'm not a security person, my thought is that forwarding in any situation is incredibly unsafe. If a criminal compromises someone's host account he could start hacking that person's accounts. He could issue password recovery requests intercepting correspondence that would alert the victim and pass on the rest. The victim would remain none the wiser since he'd get his normal deluge of emails.
If I'm correct, no one should use any sort of email forwarding.