Never made sense to me, why throw away and also why add water instead of just flour?
It's only really necessary to discard during the development stage of a starter. This has two effects, it stops the starter getting huge before it gets to the stage of being usefully active and balanced and it helps to establish the right balance of yeasts and bacteria (or alternatively helps to discourage the ones that you don't want).
Once you have a stable and balanced starter there shouldn't be any need to discard. You take what you need to start the levain for your loaf and you replace what you take with flour and water to keep your stock happy and well fed. You have to add water to keep the hydration of the starter at or about your desired consistency whether it be fluid or stiff.
Keep on bakin'