Base on my own research and further understanding, I would say that the answer would be that they are important because the creature realizes upon reading paradise lost that he resembles Adam and Satan and that there are many parallels in their relationship to their creator and in his relationship to his creator.
They contrast Victor’s creation with God's creation of man.
Explanation:
Mary Shelley made many references to Milton's "Paradise Lost" in her book "Frankeinstein", that in Shelley's conception Adam the creation of God in "Paradise Lost" is represented by the concept of creation of the monster, which is also called Adam. The connection between the references is established when Shelley shows that, like God, Frankestein created a living being in his image and likeness and had plans for this being to be perfect, at least in the eyes of its creator.
