The figurative language used in Mary Shelley's Frankenstein serves two purposes. First, the novel is a Romantic one. The Romantics adored figurative language--mainly using imagery and personification to elevate the beauty and importance of nature. Second, Mary Shelley's novel would not meet her own personal requirements without figurative language.
In her 1818 preface to the novel, Shelley states the following.
One which would speak to the mysterious fears of our nature, and awaken thrilling horror—one to make the reader dread to look round, to curdle the blood, and quicken the beatings of the heart. If I did not accomplish these things, my ghost story would be unworthy of its name.
Therefore, the use of vivid language and images would be the only way with which Shelley could insure that the reader's "fears of [their] nature" could be accomplished. The purposely limited descriptions of the Creature allow the reader to create the most frightening image possible (only limited by his or her own imagination). Outside of the limited imagery of the Creature, Shelley's novel offers readers vivid images of the nature and Victor's fears (manifested normally by the Creature).
The rain pattered dismally against the panes, and my candle was nearly burnt out, when, by the glimmer of the half-extinguished light, I saw the dull yellow eye of the creature open; it breathed hard, and a convulsive motion agitated its limbs. Chapter 5
Shelley, further in her preface, compounds the idea of her desire to illustrate the necessity of vivid images.
I see them still; the very room, the dark parquet, the closed shutters, with the moonlight struggling through, and the sense I had that the glassy lake and white high Alps were beyond. I could not so easily get rid of my hideous phantom.
She wanted each reader to see the surroundings of her own personal nightmare in the same way she did when she had the nightmare. Essentially, without the use of figurative language (like imagery), Shelley would not have been able to "frighten my reader as I myself had been frightened that night!"