I'm southern, let me help. "Y'all" refers to a group of people. It usually means the group of people that the speaker is speaking to, but you can also define the group, like "y'all in the other room" or "y'all Canadians".
"All y'all" refers to the entirety of that group, as opposed to a subset of a group, which would be "summa y'all", or none of a group, which would be "nunna y'all".
I’m Philly and we like to say you’s guys’es instead of y’all and we’ll say all’s you’s guys’es for all y’all . And jawn for that thing that can’t remember the word for but you know what I’m talking about because of the context of what I’m saying. You’s guys’s know what I’m saying.
It’s for when you’re referring to more people than every single person that you’re referring to.
See: ATM machine, a machine that dispenses automated teller machines; PIN number, the number that must be used in order to use a personal identification number; NIC card, a network interface card that a NIC uses to access a network interface.