And more importantly, what does it matter?

Have you ever noticed a tomato sauce that just doesn't quite taste right? It's not bad, but it seems a little flat. Not quite tomato-y enough. Often, you'll discover that that tomato sauce was made without wine, or vodka, or any of the other alcohols frequently used in cooking tomatoes. Is this just because I enjoy cooking with and tasting alcohol? Actually, it's not. There is a very specific reason to use alcohol when you cook tomatoes.

There are a number of flavor compounds in tomatoes that are only dissolved (and therefore released) in alcohol. So while you can certainly cook tomatoes without alcohol, you will not get that big tomato flavor you're expecting. That's the reason behind, for example, penne alla vodka, a delicious pasta presentation that became very popular in the 80s and seems to be making a big comeback. It's not that you can taste the vodka — it's that the vodka releases all of these flavor compounds in the tomato sauce. (And there really isn't even very much vodka in the recipe!)

Most spices and many herbs are fat soluble, which is why we often toast or "bloom" them in oil. And if you've ever tasted a totally fat-free meal, you're not just missing the fat, you're generally also missing the powerhouse flavors that the fat releases and carries through the dish.

The flavor compounds in alliums (onions, shallots, scallions, garlic, etc.) are water soluble, but also react to fat and alcohol.

You can certainly deglaze a pan used to cook meat with water or stock, if you don't want to use alcohol, but you won't get the same flavor punch you'd get if you used wine. 

There are some food scientists who disagree with this whole notion, and suggest that the enhanced flavor profile is from the flavor of the oil, fat, or alcohol used. All I can say is that, while not claiming to be a scientist (and even admitting I wasn't very good at science in school), I have cooked for decades. And, as a recipe developer, I pay attention when I cook. I make many dishes multiple times, with small but specific variations. And, even using a tiny bit of vodka, an alcohol with virtually no taste, I find the inclusion of the alcohol creates a significant change in the flavor of certain completed dishes.

Because of this very unscientific method, used and refined over many years, I'll stick to my guns and use fat and alcohol to open up and release flavor compounds from food. Water or stock can be substituted, but be aware, there really is no "substitute." I'll admit that it is not absolutely necessary to know all of the science when you go into the kitchen… but knowledge never hurts.