"One balmy evening, I was driving down the hill from my home at roughly twenty miles per hour over the speed limit. It is a rugged area with no cross streets and one that invites disregard of rules. On this occasion, however, several cars driving up the hill flashed their lights at me... Then, from the dark recesses of my mind, a memory emerged. Flashing lights by passing motorists warn of a policeman in the area. I slowed my car, passed the policeman at the legal speed limit, and avoided a citation. That's how you change a reading."
The passing motorists are the Tarot cards, benevolently warning that danger lies ahead if you stay on your current course and change nothing. You may choose to ignore this warning, but when you pass the policeman you will pay quite literally and quite severely. Or you can, as Ms. Kliegman did, take action and change the future. In the world of the fortune-tellers, the policeman will know you were speeding and pull you over even if you reduce your speed before passing his car. Does this not sound absurd?
The Tarot does not tell fortunes because giving messages about an immutable future is counter-productive to its purpose. The Tarot is supposed to make you think, make you act, and most of all, make you learn. It does not tell you what you already know because that would not make you think. It does not tell you that nothing can be done to change a situation because that would not make you act. And it certainly does not tell you irrelevant or trivial information, because that would not teach you anything!
Now we may answer the question of what the Tarot can tell you. In questions about the future, it shows you what will happen if you change nothing and keep doing things the way you have been doing them. If the cards see danger ahead, by taking action you can avoid the danger before it can strike. If they predict good times, you must still make a conscious effort to continue doing what you are doing now, or the prediction will go unfulfilled! Tarot cards never tell you what is going to happen - they tell you what it seems will happen, based on your current situation.
Here's an analogy that I find very useful. Life is a wheel, always turning at the same speed and in the same direction. You are standing on a point somewhere on the circumference of that wheel. If you know what time it is, and how fast the wheel is turning, it's a simple mathematical calculation to find out where you will be standing at some point in the future. Tarot cards are the calculator. And if you don't like where you are standing, you aren't glued to the wheel, so can take a step backwards or a step forwards and change the calculation completely.
This operation works the other way too; if you know where you are then you can find out where you have been, as well as where you are going. Thus the Tarot cards can answer questions about the past as well as those about the future. And it can reveal the present as well. Often the cards can show the thoughts, feelings and motivations of the questioner, and of others who relate to the question. Most relationship spreads have positions for both partners for this reason. If there is something that you must know in order to solve a problem, the Tarot can and will reveal it to you. Often, you'll find that you already knew it, but you overlooked it or chose to ignore it.
The Tarot cards are also a wonderful source of advice and guidance. They can tell you not only where you are and where you are going, but also which route to take to get to your destination and when to start walking to get there on time. The guidance of the Tarot is so valuable because it comes from within, from the Inner Voice, and that is the best guidance possible. In the hands of a master, the Tarot cards can make a better counsellor than someone with years of psychological training. It is often far more accurate as a counseling tool than a divining tool. (Perhaps this is why there was a certification test for Tarot Counsellor, but not for Tarot Diviner!)
Sometimes the Tarot cards do not seem to answer the questions asked of them. This can be for one of two reasons. First, you may not have phrased your question well, and the reader misunderstood. If the reader misunderstands, the reading may still be accurate, but the reader's interpretation of it will not be. If the question was well phrased, the second possibility is that the question you have asked was not the one you wanted answered. The Tarot gives you the answer that you need to know at this time in your life. If you are asking one question and avoiding another... well, suffice it to say that a Tarot reading will often put an end to that!
With this in mind, the function of the Tarot cards can be summarized in two sentences. They do not always tell you what you want to know, but they do always tell what you need to know. They do not always answer the question that you ask, but they will always answer the question that needs answering the most.