What does the subplot with Felix and Safie allow the author to do?

The Felix and Safie story is sort of a foil to the creature's loneliness and shattered dreams. Safie is not merely brunette, but has "shining raven hair." This sort of diction elevates seemingly ordinary events to the level of the spectacular: it reveals the extent to which the creature idealizes the cottagers and all that is associated with them. He worships them, and longs for their love and acceptance. The creature's essential humanity now becomes clear to the reader: he feels sympathy, affection, and desire; he is capable of aesthetic appreciation (as we see in his enjoyment of the family's music); he has mastered language; and he is capable of self-analysis and reflection.