Without the inclusion of chapter five, in Mary Shelley's novel Frankenstein, the story presented in the novel would not exist.
Up to chapter five, Victor has told readers the tale of his life, about his obsession with science, and his desire to reanimate life. In chapter four, Victor's creation of the being comes to a head. He has begun gathering the parts necessary for his creation to live--choosing them for their symmetry and proportions.
Chapter five is crucial because it is the chapter where Victor's toils are recognized as successful. In this chapter, his creature comes to life.
I saw the dull yellow eye of the creature open; it breathed hard, and a convulsive motion agitated its limbs.
Victor's horror at the appearance of his creation fuels his abandonment of the monster. No longer in love with his creation, as he hoped, Victor is (instead) mortified by the being.
How can I describe my emotions at this catastrophe?
Instead of being the father his "child" would idealize, ("A new species would bless me as its creator and source; many happy and excellent natures would owe their being to me. No father could claim the gratitude of his child so completely as I should deserve theirs."), Victor realizes the error of his experimentation.
Therefore, without the creature coming to life in chapter five, the remainder of the story could not exist. Victor would not have abandoned his "son," his "son" would not have murdered William, and Victor would not have made it his mission to end the life of his "son." Essentially, the story would not be the tale of horror Shelley desired it to be.