You ain't no Human
What is the need of "no" here?
isn't it already meaning same without "no"?
As has been mentioned in the comments, "ain't" is non-standard English. It is therefore informal. Some dialects (such as the Southern dialect of American English) use "ain't". These dialects (including Southern) also use double-negatives as intensifiers instead of as logic puzzles.
In the original poster's example, the word "no" acts as a determiner. It therefore affects the meaning of the sentence.
The following sentence means "You are not in the category called 'human'" or "Some part of you is not made of human tissue, or is not organized the way human tissue is organized":
You ain't human.
The following sentence compares "you" to each possible individual human, and concludes that you are not one of them. It also clarifies that "you" is singular:
You ain't no human.
It emphasises a negation.
It's a double negative. The use of negatives in this way is called a Negative Concord.