There are several theories I have that are relevant to this issue.
Note of definition: I use the term "spiritual force" a lot throughout this discussion; by this I mean your personal deity, unconscious mind, universal collective consciousness, telepathic spectral gnomes, or whatever you feel guides the workings of divination media, be it actually divine or psychological.
1. Omniscient Spiritual "Force" (Universal Determinism)
As long as you don't know the order of the cards, it doesn't matter if you physically shuffle them, since the spiritual "force" that gives you the cards you "should" get takes all possible variables into account without respect to time. So the "force" knew when you last put your cards away that the next time you used them, you'd need the cards in the order they were left. This points to a formulaic determinism in the workings of the universe, which calls into question the concept of "free will," but that's another discussion.
2. Ominiscent Spiritual Force Sans Determinism - Schrodinger's Cat (Mechanism of Action I)
As long as no one knows the order of the cards, each card is in all 78 possible states at once. Not until you observe the card by turning it over do you know what it is. This is sort of a "quantum" view of Tarot. By this theory, as long as you don't know what the coming card is, it could be any of them...leaving room for a spiritual "force" (I prefer the "spectral gnomes" view of that force for this example) to change it to fit the reading at hand.
3. Omniscient Spiritual Force Sans Determinism (Mechanism of Action II) (Author's Pick)
Your omniscient spiritual "force" knows what cards you need and tells you which card to draw when you fan, or guides your hand in where you select to cut (or how many times to cut), such that you get the cards you should get. This would also apply when shuffling - the feeling that tells you when to stop. Or, you might suddenly feel inclined (or be hired) to read about a question for which the order in which the cards were left is appropriate. This applies by extension to multiple questions within the same reading without shuffling - the specific questions asked by the querent, and the order in which the querent asks, are guided by internal feelings on his or her part that may be inspired by a spiritual "force."
4. Energy Transfer (Author's Least Favorite)
By shuffling the cards, you impart to them some not-yet-understood aspect of your personal energy, which temporarily grants them sentience. The cards then take on a life of their own, choosing which cards to give you. By not shuffling before the reading, you are not endowing the cards with your current energy, and so you won't get an accurate reading for the question at hand. You might get nonsense if the cards have no energy, or you might get a reading relevant to the last energy they were given.
5. Intellectual Analysis Yields Positive Results Regardless
It doesn't matter what cards you get, you're going to get the right message from them regardless, since you apply your own personal intuition and mindset to the reading. Even if you get what seem to be wholly irrelevant cards, you'll find some relevance in them just by going through the exercise of analysis. "If you look hard enough, you'll find what you're looking for."
6. You're a Chicken Tarotist, Don't Worry About It!
Excuse the DuQuette-ism in the title of this one!
Who cares how it works? If shuffling or not shuffling works for you, do that. Don't worry about traditions or superstitions...just do what you feel you should. If you are happy with your results, then you're doing it right. That's what matters.
I strongly favor theory 3 in my personal applications of Tarot, but many others choose 1, 2, 4, 5, or some other theory for how the cards "work." In any case, 6 is still good advice! Do what works for you!
(Apologies in advance if I wasn't as clear, concise, or complete as usual in my ideas above...I've got this horrible cold and I can't concentrate!)