Two reasons! First, this grain is high in phytic acid, an anti-nutrient that blocks the absorption of vitamins and minerals in the body. In other words, if you eat foods with a lot of phytic acid, you won't actually gain much nutritional value from them. Soaking the grain in water with lemon juice or apple cider vinegar for 8+ hours neutralizes the phytic acid, so your body can use all those good vitamins and minerals in the millet. Second, soaking the millet makes it easier to blend and achieve a creamy porridge consistency. It also reduces the cooking time.