This is the reason from baking911.com:
Steaming the dough as it bakes also gelatinizes starch on the outside layer, producing a bread with a crisp crust and a brown crust color in varying degrees; too much steam results in an undesirable crust. Steam also helps to prevent wild breaks in the loaves because it delays the setting of the bread's crust, allowing it more time to bake and brown. However, in contrast, during the last stages of baking, a dry oven is required when the crust is browning; after the steam is removed, the gelatinized layer dries out forming a thick crunchy crust.
Among various articles I've read, the best way to add the moisture is in the form of ice cubes set in a pan below the bread in the hot oven. Get the temp set, add the bread, then the pan below it. The high heat will quickly melt the cubes and give it the needed burst of moisture, then it will evaporate quickly so the latter baking will be the needed dry time.
Commercial ovens have built in steam jets set on timers.