Why did Gothic literature appear; what was the importance of it?
The first Gothic novel is indicated to be Horace Walpole's 'The Castle of Otrantro", written in the mid-late 1700's. This novel was the first of its kind that combined all the elements often associated with the Gothic novel which include: The infiltration of the supernatural, a castle or medieval setting, darkness, an ancient curse, the idea of the inevitable, mystery, melancholy, coldness, sadness.
After Walpole many people began to imitate his style, and by the 1800's, particularly the mid 1800's, Gothic literature was all the rage in the works of Mary Shelley, Oscar Wilde, Edgar Allan Poe, and many other authors.
It came as a reaction to all the rational\critical works produced during the 17th century (The Age of Enlightenement\reason) by critics such as: Alexander Pope, Edmund Burke, Voltire, & others. People wanted something that expresses their feares and show the irrational side of their inner selves. It started on the Romantic period (as mentioned above).