What is the main purpose of a thought bubble in a graphic novel......................................................it illustrates internal dialogue?

Question: What is the main purpose of a thought bubble in a graphic novel? it displays authentic dialogue. it shows what the characters say. it gives background information. it illustrates internal dialogue?

Bubbles drawn with dotted lines mean that the character is speaking quietlty. White spaces between each panel are called a gutter and there can either be horizontal or vertical gutters. Rounded or irregularly shaped outline as in a cartoon or other drawing containing words that represent a characters thoughts and often featuring several smaller rounded outlines that form a tail pointing toward the thinker.