Great question — answering as the UphillTech team:
In a dry oven, a dry crust forms relatively quickly on the surface of the cake. Steam delays this crust formation, keeping the surface wet and allowing more moisture to evaporate from the interior of the cake over the extended baking time. Speaking of time, because steam keeps the surface moist, it takes longer for the cake to "set" and test done with a toothpick. Baking for longer = more moisture is driven off from the cake's interior. Steam also changes how starches gelatinize. It gives that chewy texture that is great for bread. In cakes, you want a tender crumb, and the altered starch behavior with steam can lead to a tougher, drier texture. You saw that with your olive oil cake. The...
Full answer by Luciano: https://cooking.stackexchange.com/a/133087 (CC BY-SA 4.0).
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