I'll reiterate my answer to your other, very similar question
As in many languages, in English commas represent the pauses that a speaker would include when saying the sentence out loud. In this way it mirrors the natural rhythm of the language -- which of course varies considerably between different dialects and different individuals.
So most rules that seem to require commas before or after certain words or phrases are simply guidelines to help you organize your English sentences. It's a good idea to learn these guidelines, and understand what purpose they serve, but then pay attention to how native speakers talk and write, and develop your own personal style.
It is entirely up to how the author wants the reader to perceive the flow of the sentence, as if it was spoken. If I wish to write a long sentence with a lot of detail and make it sound like I was saying it all in one breath I would omit the comma between two independent clauses.
But, I'm the sort of writer who likes significant pauses, so I don't.