It's really good when you host small Web sites and don't want to fiddle around much. Especially cause I can setup resellers who can manage their own accounts. Agencies use it a lot. There whole business can run on a couple of servers running cpanel with WordPress on each account.
I would assume yes. I think its niche is the entry-level layman. Which is probably why a HN reader wouldn't encounter it very often.
Yes. It's standard on most shared web hosts.
Yes, pretty heavily. If it is a marking/small business website there is a very good chance it is running on cPanel.