Tales of the Cthulhu Mythos

I'm curious about the creative process behind selecting certain myths or stories. Did the author aim to fill in gaps or offer fresh perspectives on well-known tales?

The atmosphere was so immersive that I kept wondering how much of that was intentional. Was the slow build part of the design to heighten the sense of unease?

Adding original tales alongside classics gave a sense of expanding the universe. I wonder what the author's goal was in blending old and new in this way?