That’s an excellent question, Paul C. from Montreal! I was planning on writing about that, back when I was running my fingers about Viennese ovens, but forgot. In fact steam does a lot of things.
When steam settles onto a loaf of baking bread, it keeps the surface of the loaf moist and stretchy. That allows the bread to expand, producing a lighter crumb. Second, it cools the loaf’s surface, giving enzymes time to continue their work of breaking long-chain carbohydrates down into simple sugars. Those sugars improve the flavor of the crust, but they also do something else: they caramelize in the heat. Caramelization produces new flavors, plus a toasty brown color.
That color itself, you might be surprised to discover, also plays an important role in making a crust crispy. For darker colors, as you’ll remember from your high school science classes, are more efficient at absorbing heat. Which means that the darker the crust gets, the more heat it absorbs, which makes it darker and hotter still, drying it out. Thus the exterior becomes nice and crackly-crispy. Moisture introduced at that point would ruin that nice dry exterior, which is why you only want to employ steam in the early stages of baking.
Brick ovens, when sealed properly, do a fine job of retaining moisture. Where does that moisture come from? Why from the bread of course, which is why a brick oven full of bread produces better crust than a mostly empty one (and why I buy bread flour in 50-pound bags). Most home ovens (specifically gas-powered models) don’t do a particularly good job of retaining steam, mostly because they have to vent to allow the waste products of natural gas combustion (like CO2) to escape.
You can add it back manually of course, which is why many home bakers spritz the insides of their ovens with a spray bottle of hot water. Modern deck ovens have special steam nozzles that introduce copious amounts of preheated water into the baking area, producing the most steam-intensive environment of all. For a fast-rising, fast-baking loaf like a baguette, it makes all the difference, creating a thin, crackling crust instead of a thick, hard and crunchy one. Which isn’t bad, just not technically ideal for a baguette.