Imagine we are the cup.
When a cup is empty, you could fill it with either garbage or something edible. Of course, as the owner of the cup, we could decide which would go in. If we discovered that we have placed garbage in our cup, we could also decide to pour the garbage out. But unless you wash it, there would still be traces of the garbage and that would not be nice when it is filled with something great. Therefore, it should be washed with water.
Being empty can be a humbling experience. Just decide what to be filled with.