Because all though he knows his sister is dead, he can still hear knocking and scratching sounds coming from her coffin. She's dead, but comes back to life to haunt him causing his madness to grow stronger.
Extra words in a sentence that add detail and are set off by commas are called nonessential elements. This is because they are not essential to the sentence and story, they are simply an added bonus, without them the sentence still makes sense.
