Well, this is a question about something what is truly my headache, for a long time already. I can't get if one should use a comma in sentences like those below, or not. I’ve heard from a native speaker that a comma is no need here, that it’s a mistake. Actually, although the sentences without a comma looked weird to me, I believed him. I saw such punctuation very often on forums, but what was it: an illiteracy or just a rule of English language? I couldn’t know.
Ok, recently I was watching a movie in English, with English subtitles, and all the addresses were marked out with commas there.
So, it is a torture for me!!!!

Could someone please just tell me if a comma is needed here, or not?

Daddy, I want this toy.

I want this toy, daddy.

John, can you go to fetch bread?

Yes, sir.

Yes, my dear.

John, I forgot to take the camera.

You know, John, I forgot to take the camera.

Let's go, my friend.

John, could you be more polite?

Thank you, John.