the time has been, That, when the brains were out, the man would die, And there an end; but now they rise again, With twenty mortal murders on their crowns, And push us from our stools: this is more strange
Thank you for the opportunity to help you with your question!
Shakespeare used these lines to be somewhat comical. Maybe not in the comical sense that you would think of, it's more of a dark comical way of explaining things. These lines were important because of Macbeth's shock to seeing a ghost. If you were to paraphrase these lines it's like he was saying at one point in time when you kill someone they are really dead, but now even in death a person can still not leave you alone. It was very important for the lines to let us peek into the true reaction of the character.
Please let me know if you need any clarification. I'm always happy to answer your questions.