A cpanel is a webpage access to the lines you have from your provider, they give you the ability to restart you line, add new lines for friends and family and extend your line, all without having to contact your provider. Some providers do offer them and others don't. If its a feature offered I don't see why you should have to pay extra as if your line provider is good and the lines stable you should never have any issues so wouldn't need to restart your line and I think its good to have an open line of communication with your provider anyway.
If you need lines for friends and family then just drop your provider an email or pass over their details to those who want the line so they can have communication with the provider. I'm sure someone will post on the positives on having a cpanel so it really is a personal choice.