Hair, just like any living thing, needs water to thrive. There is a grand difference between hydrating the hair and moisturizing the hair. Keeping the hair at an optimal hydration and moisture level is key to keeping the hair soft, healthy and flourishing. The only way to hydrate the hair is by water. It is best to completely saturate the hair in water, under a shower head or at your favorite stylists shampoo bowl. Saturating the hair with water gives the hair follicles the chance to open up, that way the water is actually entering the hair instead of just sitting on top of it. This is why it is so important to wash the hair weekly, or at the very least biweekly. A spray bottle filled with water is not adequate enough to hydrate the hair in the way that it truly needs. Now, moisturizing the hair locks the hydration in place. Without the moisturizing process, the water that we just worked so hard to get in to the hair, would simply evaporate. Following up your shampoo with a deep conditioning treatment then a leave-in conditioner, helps to make the hair soft, maneagable, moisturized and it helps the hair to stay moisturized until your next wash day.
It may seem complicated, but the moral of the story is our hair needs water. But, in order to keep that water in our hair, we have to follow up with a moisturizing product such as a leave in conditioner/curl milk/etc. This will help to keep the hair healthy and prevent breakage/split ends and promote length retention.