I don’t hear it ever, and I wouldn’t interpret it as being for emphasis. I’d interpret it as a lower register dialect as an unconjugated variant of ‘to say’ pronounced: I sez, you sez, he sez, we sez, they sez

It sounds very natural to me for relating conversations. I'd use it in every day speech with friends. I wouldn't use it in formal situations.

I've never heard "I Says". From time to time I've heard "says who?" meaning, by what authority is this order being given? People might respond, "says me", meaning i have sufficient authority to issue this order, etc.

As an American, the words "I says" are common within some geographic regions and economic groups. This is a colloquial way of speaking, and you will never need to use it. But it is good that you understand it. The situation that I hear this in most often is when someone is telling a story that happened in the past, but they are telling the story as if it is happening now. For example: "So my boss calls me into his office and I says to him 'You wanted to speak to me, boss?'" Good luck!

like "arigato kusamis"