This shows the extend of Marc Antony's grief, because it brings the reader the image of his heart having been torn out of his own chest and buried with the body of Caesar. It not only shows his grief (the image of his heart being dead), but it also shows how absolutely he loved Caesar. If you give your heart to someone it shows that you have a great amount of affection for them. He is showing that with this passage; he has given his heart to Caesar and it is about to be buried with his body.
