Tarot works with a series of images and symbols, interpreted by the tarot reader. In every tarot reading there is the reader (in this case, me) and the querent. The querent is the customer or the person seeking answers.
Many people believe that it works based on Jungian archetypes and the collective unconscious. Because we all share the same "mind" I am able to intuitively read the images and present the querent with answers that resonate.
Other people believe that the cards are selected and arranged by supernatural forces acting through the reader, and the these supernatural forces also guide the reading.
I believe most tarot readings are a combination of both. There are readings where I feel a strong connecting to the querent and am simply able to intuit the answers they are seeking. Other times I feel a strong push or pull coming from outside forces. Sometimes these forces are coming from my world or my life, and sometimes they are coming from the querent. Some people arrive at their reading surrounded by very strong supernatural energy. I'll bet many people don't know that they go through life followed around by supernatural beings.
Supernatural beings exist for a variety of reasons. They exist as protectors and guides, they exist simply because they choose to and are curious about us, and sometimes they even exist for nefarious reasons.