Signing don't want allows baby to indicate preferences. It is most often used in the context of food, for baby to be able to decline certain foods without having to have a tantrum or throw food. You can use don't want to redirect these types of negative behavior. If baby throws food they don't want off their plate, you can redirect by saying, "You don't want your peas."
Use the don't want flash card when introducing baby to options. For example, baby may want one toy and not another. It is also useful when helping baby develop food preferences.