Can someone explain Advanced Call Forwarding to me?

What EXACTLY does setting the number of rings on Advance Call Forwarding do?  I understand that I can add number(s) (like my cell phone) to have it ring on the other number(s) simultaneously but 'forwarding' to my home number makes no sense to me.

Why, if I set the "STATUS" to "OFF" on my home phone number do I get a warning stating that my home phone will not ring.  That sounds counter intuitive. 

Does this setting have anything to do with the number of rings before the call is transferred to voicemail? I don't see any place where such a setting exists.