What is the main purpose of a speech bubble in a graphic novel?

Speech bubbles in comics or graphic novels are essential tools that are used to convey dialogue that is being uttered aloud. They are one of the most important elements because they reveal what an author is going to depict in a story and how characters vary from one another.  

The bubble is also depicted in different shapes indicating someone may be yelling (pointy), thinking (cloud-shaped), whispering (dotted lines), or speaking in a manner different than the other characters (different font, bubble shape).

It illustrates internal thoughts.

Explanation:

in the quotation from passage, connie willis states that literature is a way for authors from the past to communicate with readers of today. we can read their stories, books, and other texts to learn lessons about life. willis refers to these lessons as messages “from beyond the grave.” willis has a favorable viewpoint about the timelessness of literature.

tom robinson he was a black man that was accused of raping caucasian woman, and an idealistic lawyer offers to defend him. which makes nobody believe in the innocence of a negro.