|Most people who use a Tarot deck do so for giving readings or divinations, or for what some people would call "fortunetelling." How does the Tarot work for these purposes?
I think we can pretty well ignore the "Demons control it" aspect simply because there is absolutely no evidence to support it. The answer most frequently given today is "synchronicity." Unfortunately, most people don’t understand the concept of they have an errant notion of what it is. The truth is that synchronicity is nothing but a word and tells us nothing.
The popular use of the term was created by the psychologist Carl Gustav Jung (1875??"1961). Jung was the son of a priest. His grandfather is alleged to have been the illegitimate son of the German philosopher, poet, and mystic Goethe. At first, Jung was a student of Sigmund Freud, and eventually became a coworker. They often disagreed over each other’s theories, and Freud attempted to move Jung away form the occult on different occasions. They eventually discovered some sexual secrets about each other: Freud was sleeping with Jung’s wife and Jung was sleeping with one of his young female patients. They agreed not to reveal these secrets and soon went in different directions with their psychological theories and practices.
In his book The Structure and Dynamics of the Psyche, Jung first observed that there wee "meaningful coincidences" in life. A common example of such a meaningful coincidence would be getting a phone call from someone you haven’t heard from in years just moments after thinking about that person. He called this experience a "synchronicity." It is important to note that Jung considered synchronicities to be "acausal." That means there is no cause-and-effect relationship between the parts of the synchronicity. Your thinking about a person did not cause the person to call you. It is just a "meaningful coincidence."
The key concept here is "meaning." Meaning is an interpretation we put on something. For example, if I were to say the word blue, you would think of a certain shade of color, perhaps that of the sky on a bright, clear day or maybe the dark-blue hue taken on by the sea at nightfall. By itself, the word blue is just a few sounds. But when we hear it, we associate it with ideas. We give words meaning.
A coincidence is simply a series of events that is accidental, but appears to be planned or arranged. For example, I currently live in southern California. Instead of the traditional four seasons, we actually have but two: a short wet season and a long dry season. If I wash my car several times during the wet season and it rains within a few days of the wash, it would be a coincidence (and not a difficult one to understand) that one followed the other. If I were to say, "Every time I was my car it rains," I would be implying that one cause the other. Of course, they would be unrelated; one thing following the other would be just a coincidence. However, I could try to find a cause for the coincidence. If such a cause were discovered, it would no longer be a coincidence. The effect would have a direct cause.
We now have three concepts. The first is a coincidence, wherein two or more events seem related but are not. The second is the notion of "acausality," where something (B) follows something else (A), but A does not cause B (even though they appear related). The third is the concept of meaning, which is simply that our minds give a value to something.
Synchronicity, or meaningful coincidence, is the idea that two or more things have absolutely no cause-and effect relationship to each other, but our minds give meaning to it. In short, we create meaning from nothing. A synchronicity consists of events that happen at random, that have no relationships to each other, but to which we give meaning.
Therefore, as an explanation for how a Tarot reading works, synchronicity is a horrible failure because it implies that giving a Tarot reading works not because the Tarot is in any way whatsoever related to your life or problems, but because the cards fall, by sheer chance, in a particular way that the reader finds meaningful. To give a true answer as to why the Tarot works, you would have to find out why the cards fall in a particular way. Synchronicity is a description, not an answer.