The water isn’t added for heat—it’s there to create humidity, which prevents the dough from forming a skin. In a sealed oven, the humidity generated from the initial set up lingers for a significant amount of time because the oven is cold, so there’s no dry heat to evaporate it, even as the water cools.
Humidity can persist even under high heat, which is why you get such strong oven spring when baking in a Dutch oven. That’s also why the lid needs to be removed after 20 to 30 minutes—to allow the steam in the Dutch Oven to evaporate from the dry heat so the crust will develop.