Steam carries heat much better than dough does, so when steam is present, it allows the dough’s surface to become much hotter, much quicker than it otherwise would. This means the dough heats up much faster and can rise much higher and quicker during the initial phase of baking before the crust sets.
Your bread has roughly a 15 to 20 minute window of opportunity to rise before the crust is set. Giving bread the highest heat possible at this initial baking time will ensure that the gasses can force the bread to increase in volume as quickly as possible during this time, giving your sourdough the best chance of a good oven spring.
The problem with most home ovens, is that: 1. Steam escapes from the oven quite freely 2. Heat escapes through the door, but especially when the door is opened to put your bread in.