In countries where water is in short supply it has to be carefully and sparingly used. By extension the proverb means that you should not discard anything until you have made sure of a replacement. For example, don’t through away the stump of a pencil until you have provided yourself with a new one; if you wish to jot down an important note while it is fresh in your mind, it is better to have the stump than no pencil at all.
