It doesn't really keep them from getting "dried out". That's where a suitable ambient air humidity level and substrate moisture come into play. Those two aspects along with drinking/soaking help to ensure that a tortoise, especially a young one, remains well hydrated. Soaking gives them another opportunity to drink water along with the benefit of usually encouraging wastes to be expelled that they would otherwise be held longer until water is available next. They should still have drinking water available in their enclosure to be able to drink when they want to also. Soaking is just added insurance.