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:

hygelac - beowulf's uncle, king of the geats, and husband of hygd. hygelac heartily welcomes beowulf back from denmark. hygd - hygelac's wife, the young, beautiful, and intelligent queen of the geats. hygd is contrasted with queen hope this