Question

. . . and at length found myself, as the shades of the evening drew on, within view of the melancholy House of Usher. I know not how it was—but, with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit. —Poe, "The Fall of the House of Usher"

In the bolded section, what is the benefit of using "I know not…" instead of "I don't know" or another more concise syntax?